Entering their fourth year of war against Japan, Chinese military forces were strengthening their air force, producing their own armaments, and training their officers in the methods of modern war. Here, Chinese cadets in full battle dress, they favor the German type of steel helmet, on parade somewhere in China, on July 11, 1940.

Entering their fourth year of war against Japan, Chinese military forces were strengthening their air force, producing their own armaments, and training their officers in the methods of modern war. Here, Chinese cadets in full battle dress, they favor the German type of steel helmet, on parade somewhere in China, on July 11, 1940.

On April 9, 1940, Germany simultaneously invaded Norway and occupied Denmark, and the war began in earnest. On May 10, German forces swept through Belgium and the Netherlands in what became known as “blitzkrieg,” or lightning war. Three days later, Hitler’s troops crossed the Meuse River and struck French forces at Sedan, located at the northern end of the Maginot Line, an elaborate chain of fortifications constructed after World War I and considered an impenetrable defensive barrier. On June 14, German forces entered Paris; a new government formed by Marshal Philippe Petain (France’s hero of World War I) requested an armistice two nights later.

From the end of 1940 through the summer of 1941, the conflicts among nations grew into true World War. The East African campaign and Western Desert campaign both began during this period, with largely Italian and British forces battling back and forth across the deserts of Egypt and Libya and from Ethiopia to Kenya. The Tripartite Pact – a declaration of cooperation between Germany, Italy, and Japan – was signed in Berlin.

Japanese forces occupied Vietnam, established bases in French Indochina, and continued to attack China. Mussolini ordered his forces to attack Greece, launching the Greco-Italian War and the Balkans Campaign. The Battle of Britain continued as the forces of Germany and Britain carried out bombing raids and sea attacks against each other. The United States began its lend-lease program, which would eventually ship $50 billion worth of arms and materials to to Allied nations. And an ominous new phase began as the Germans established walled ghettos in Warsaw and other Polish cities, rounding up Jews from surrounding areas and forcing them to move into these enclaves. Here are a collection of interesting and rare pictures documenting this period of World War II.

British Infantrymen in position in a shallow trench near Bardia, a Libyan Port, which had been occupied by Italian forces, and fell to the Allies on January 5, 1941, after a 20-day siege.

British Infantrymen in position in a shallow trench near Bardia, a Libyan Port, which had been occupied by Italian forces, and fell to the Allies on January 5, 1941, after a 20-day siege.

Against a background of a rock formation, a British bomber takes off on May 15, 1941, from somewhere in East Africa, leaving behind a trail of smoke and sand.

Against a background of a rock formation, a British bomber takes off on May 15, 1941, from somewhere in East Africa, leaving behind a trail of smoke and sand.

Warships of the British Mediterranean Fleet bombarded Fort Cupuzzo at Bardia, Libya, on June 21, 1940. On board one of the battleships was an official photographer who recorded pictures during the bombardment. Anti-aircraft pom-pom guns stand ready for action.

Warships of the British Mediterranean Fleet bombarded Fort Cupuzzo at Bardia, Libya, on June 21, 1940. On board one of the battleships was an official photographer who recorded pictures during the bombardment. Anti-aircraft pom-pom guns stand ready for action.

An aerial view of Tobruk, Libya, showing petrol dumps burning after attacks by Allied forces in 1941.

An aerial view of Tobruk, Libya, showing petrol dumps burning after attacks by Allied forces in 1941.

Bardia, a fortified Libyan seaport, was captured by British forces, with more than 38,000 Italian prisoners, including four generals, and vast quantities of war material. An endless stream of Italian prisoners leaves Bardia, on February 5, 1941, after the Australians had taken possession.

Bardia, a fortified Libyan seaport, was captured by British forces, with more than 38,000 Italian prisoners, including four generals, and vast quantities of war material. An endless stream of Italian prisoners leaves Bardia, on February 5, 1941, after the Australians had taken possession.

A squadron of Bren gun carriers, manned by the Australian Light Cavalry, rolls through the Egyptian desert in January of 1941. The troops performed maneuvers in preparation for the Allied campaign in North Africa.

A squadron of Bren gun carriers, manned by the Australian Light Cavalry, rolls through the Egyptian desert in January of 1941. The troops performed maneuvers in preparation for the Allied campaign in North Africa.

This armorer of the R.A.F.'s middle east command prepares a bomb for its mission against the Italian forces campaigning in Africa. This big bomb is not yet fused, but when it is it will be ready for its deadly work. Photo taken on October 24, 1940.

This armorer of the R.A.F.’s middle east command prepares a bomb for its mission against the Italian forces campaigning in Africa. This big bomb is not yet fused, but when it is it will be ready for its deadly work. Photo taken on October 24, 1940.

The moment a patrol of British Hurricane fighter planes, flying over a middle east sector, broke formation to attack enemy aircraft, on December 28, 1940.

The moment a patrol of British Hurricane fighter planes, flying over a middle east sector, broke formation to attack enemy aircraft, on December 28, 1940.

This photo, made from a British warplane during the assault of Tobruk shows the Italian Cruiser San Giorgio, burning amidships, in the harbor of Tobruk, on February 18, 1941. The ship was scuttled, its decks appear to be covered with wrecked and smashed gear.

This photo, made from a British warplane during the assault of Tobruk shows the Italian Cruiser San Giorgio, burning amidships, in the harbor of Tobruk, on February 18, 1941. The ship was scuttled, its decks appear to be covered with wrecked and smashed gear.

The body of an Italian soldier lies where he fell during battle, in a stone-walled fort somewhere in the West Libyan desert, on Febrary 11, 1941.

The body of an Italian soldier lies where he fell during battle, in a stone-walled fort somewhere in the West Libyan desert, on February 11, 1941.

A British Cruiser tank is unloaded at a port in Egypt, on November 17, 1940. It is one of a large number which had just been shipped there by British forces.

A British Cruiser tank is unloaded at a port in Egypt, on November 17, 1940. It is one of a large number which had just been shipped there by British forces.

Haile Selassie (right), exiled Emperor of Ethiopia, whose empire was absorbed by Italy, returns with an Ethiopian army recruited to aid the British in Africa, on February 19, 1941. Here, the emperor inspects an airport, an interpreter at his side. On May 5, 1941, after the Italians in Ethiopia were defeated by Allied troops, Selassie returned to Addis Ababa, and resumed his position as ruler.

Haile Selassie (right), exiled Emperor of Ethiopia, whose empire was absorbed by Italy, returns with an Ethiopian army recruited to aid the British in Africa, on February 19, 1941. Here, the emperor inspects an airport, an interpreter at his side. On May 5, 1941, after the Italians in Ethiopia were defeated by Allied troops, Selassie returned to Addis Ababa, and resumed his position as ruler.

Cameron Highlanders, a Scottish infantry regiment of the British Army, and Indian troops march past the Great Pyramid in the North African Desert, on December 9, 1940.

Cameron Highlanders, a Scottish infantry regiment of the British Army, and Indian troops march past the Great Pyramid in the North African Desert, on December 9, 1940.

Field Marshal Gen. Erwin Rommel, commander of the German Afrika Korps, drinks out of a cup with an unidentified German officer as they are seated in a car during inspection of German troops dispatched to aid the Italian army in Libya in 1941.

Field Marshal Gen. Erwin Rommel, commander of the German Afrika Korps, drinks out of a cup with an unidentified German officer as they are seated in a car during inspection of German troops dispatched to aid the Italian army in Libya in 1941.

A huge Panzer IV German tank, part of the German expeditionary force in North Africa, halts in the Libyan Desert on April 14, 1941.

A huge Panzer IV German tank, part of the German expeditionary force in North Africa, halts in the Libyan Desert on April 14, 1941.

Children of Japan, Germany, and Italy meet in Tokyo to celebrate the signing of the Tripartite Alliance between the three nations, on December 17, 1940. Japanese education minister Kunihiko Hashida, center, holding crossed flags, and Mayor Tomejiro Okubo of Tokyo were among the sponsors.

Children of Japan, Germany, and Italy meet in Tokyo to celebrate the signing of the Tripartite Alliance between the three nations, on December 17, 1940. Japanese education minister Kunihiko Hashida, center, holding crossed flags, and Mayor Tomejiro Okubo of Tokyo were among the sponsors.

A Japanese bomber in flight on September 14, 1940. Below, smoke rises from a cluster of bombs dropped on Chongqing, China, near a bend of the Yangtze River.

A Japanese bomber in flight on September 14, 1940. Below, smoke rises from a cluster of bombs dropped on Chongqing, China, near a bend of the Yangtze River.

Chinese soldiers man a sound detector which directs the firing of 3-inch anti-aircraft guns, around the city of Chongqing, China, on May 2, 1941.

Chinese soldiers man a sound detector which directs the firing of 3-inch anti-aircraft guns, around the city of Chongqing, China, on May 2, 1941.

With nothing but devastation confronting him, this Chinese waterboy still carried on after four days and nights of aerial bombardment at the hands of Japanese warplanes, in Chongqing, China, on August 10, 1940.

With nothing but devastation confronting him, this Chinese waterboy still carried on after four days and nights of aerial bombardment at the hands of Japanese warplanes, in Chongqing, China, on August 10, 1940.

A Japanese tank passes over an emergency bridge, somewhere in China, on June 30, 1941.

A Japanese tank passes over an emergency bridge, somewhere in China, on June 30, 1941.

This aerial view shows Japan's home fleet, arrayed in battle line, on October 29, 1940, off the coast of Yokohama, Japan.

This aerial view shows Japan’s home fleet, arrayed in battle line, on October 29, 1940, off the coast of Yokohama, Japan.

French colonial forces move out of Haiphong, in the Tonkin region of French Indochina, on September 26, 1940, as Japanese occupational troops take over the port and city under the terms of the Franco-Japanese agreement, where Vichy France granted military access to Japanese forces.

French colonial forces move out of Haiphong, in the Tonkin region of French Indochina, on September 26, 1940, as Japanese occupational troops take over the port and city under the terms of the Franco-Japanese agreement, where Vichy France granted military access to Japanese forces.

Italian bombers on their way to war action on the Albanian-Greek frontier, on January 9, 1941. Italian armies had launched an invasion of Greece from Albanian territory on October 28, 1940.

Italian bombers on their way to war action on the Albanian-Greek frontier, on January 9, 1941. Italian armies had launched an invasion of Greece from Albanian territory on October 28, 1940.

Royal Air Force bombers carry out a raid on the Italian-occupied port city of Valona, Albania on January 11, 1941.

Royal Air Force bombers carry out a raid on the Italian-occupied port city of Valona, Albania on January 11, 1941.

A squad of German soldiers pass through a Greek village, during the occupation of Greece, in May 1941.

A squad of German soldiers pass through a Greek village, during the occupation of Greece, in May 1941.

The price paid by German air invaders on the Greek island of Crete. While fighter aircraft patrolled the area, troop-carrying aircraft followed, escorted by bombers. Here, a paratroop aircraft crashes to the ground on June 16, 1941.

The price paid by German air invaders on the Greek island of Crete. While fighter aircraft patrolled the area, troop-carrying aircraft followed, escorted by bombers. Here, a paratroop aircraft crashes to the ground on June 16, 1941.

A fallen paratrooper and his parachute, on the island of Crete, in early 1941.

A fallen paratrooper and his parachute, on the island of Crete, in early 1941.

To alert their own airforce to their presence, soldiers spread the Swastika across boats used by the S.S. troops to cross the Gulf of Corinth, Greece, on May 23, 1941.

To alert their own airforce to their presence, soldiers spread the Swastika across boats used by the S.S. troops to cross the Gulf of Corinth, Greece, on May 23, 1941.

A view from the roof of St. Paul's Cathedral in London in January, 1941, showing how the famous building was ringed by fires on the night of the great Blitz. Devastated buildings are seen on every hand, with the tower of the Old Bailey, surmounted by its statue of Justice, still standing to the upper left.

A view from the roof of St. Paul’s Cathedral in London in January, 1941, showing how the famous building was ringed by fires on the night of the great Blitz. Devastated buildings are seen on every hand, with the tower of the Old Bailey, surmounted by its statue of Justice, still standing to the upper left.

The dramatic and tragic scene as the Cunard White Star liner Lancastria was sunk on August 3, 1940. The Lancastria was evacuating British nationals and troops from France, and had boarded as many as possible for the short trip - an estimated 4,000 to 9,000 passengers were aboard. A German Junkers 88 aircraft bombed the ship shortly after it departed, and it sank within twenty minutes. While 2,477 were rescued, an estimated 4,000 others perished by bomb blasts, strafing, drowning, or choking in oil-fouled water. Photo taken from one of the rescue boats as the liner heels over, as men swarm down her sides and swim for safety to the rescue ships. Note the large number of bobbing heads in the water.

The dramatic and tragic scene as the Cunard White Star liner Lancastria was sunk on August 3, 1940. The Lancastria was evacuating British nationals and troops from France, and had boarded as many as possible for the short trip – an estimated 4,000 to 9,000 passengers were aboard. A German Junkers 88 aircraft bombed the ship shortly after it departed, and it sank within twenty minutes. While 2,477 were rescued, an estimated 4,000 others perished by bomb blasts, strafing, drowning, or choking in oil-fouled water. Photo taken from one of the rescue boats as the liner heels over, as men swarm down her sides and swim for safety to the rescue ships. Note the large number of bobbing heads in the water.

German Anti-Aircraft guns belch smoke somewhere along the Channel coast of France, on January 19, 1941.

German Anti-Aircraft guns belch smoke somewhere along the Channel coast of France, on January 19, 1941.

This photograph was taken on January 31, 1941, during a nigthtime air raid carried out by the Royal Air Force above Brest, France. It gives a graphic impression of what flak and anti-aircraft fire looks like from the air. In the period of three to four seconds during which the shutter remained open, the camera clearly captured the furious gunfire. The fine lines of light show the paths of tracer shells, and the broader lines are those of heavier guns. Factories and other buildings can be seen below.

This photograph was taken on January 31, 1941, during a nighttime air raid carried out by the Royal Air Force above Brest, France. It gives a graphic impression of what flak and anti-aircraft fire looks like from the air. In the period of three to four seconds during which the shutter remained open, the camera clearly captured the furious gunfire. The fine lines of light show the paths of tracer shells, and the broader lines are those of heavier guns. Factories and other buildings can be seen below.

Two examples of Britain's war forces, a soldier in battle dress and a bearded Canadian sailor share a light at an English port, on January 14, 1941.

Two examples of Britain’s war forces, a soldier in battle dress and a bearded Canadian sailor share a light at an English port, on January 14, 1941.

Jimmy Stewart, former movie star, is sworn in as a second lieutenant in the U.S. Air Corps by Lt. E.L. Reid, personnel officer of the west coast training center at Moffett Field, California, on January 1, 1941. Stewart was one of Hollywood's most popular actors before he was inducted into the Army in 1941.

Jimmy Stewart, former movie star, is sworn in as a second lieutenant in the U.S. Air Corps by Lt. E.L. Reid, personnel officer of the west coast training center at Moffett Field, California, on January 1, 1941. Stewart was one of Hollywood’s most popular actors before he was inducted into the Army in 1941.

Outdated, but serviceable U.S. destroyers sit in the Back Bay at the Philadelphia Navy Yard, on August 28, 1940. Plans were well underway to bring these ships up to date and transfer them to Allied countries to aid their defense. These programs would be signed into law as the Lend-Lease program in March of 1941, and would result in billions of dollars worth of war material being shipped overseas.

Outdated, but serviceable U.S. destroyers sit in the Back Bay at the Philadelphia Navy Yard, on August 28, 1940. Plans were well underway to bring these ships up to date and transfer them to Allied countries to aid their defense. These programs would be signed into law as the Lend-Lease program in March of 1941, and would result in billions of dollars worth of war material being shipped overseas.

A crew of observers on the Empire State building, during an air defense test, on January 21, 1941 in New York City, conducted by the U.S. Army. Their job was to spot

A crew of observers on the Empire State building, during an air defense test, on January 21, 1941 in New York City, conducted by the U.S. Army. Their job was to spot “invading enemy” bombers and send information to centers which order interceptor planes. The tests, to run for four days, covered an 18,000-square-mile area in northeastern states.

U.S. Postal employees feed 17 tons of reading matter, labeled by postal authorities as propaganda, into a furnace in San Francisco, California, on March 19, 1941. The bulk of the newspapers, books, and pamphlets came from Nazi Germany and some from Russia, Italy and Japan.

U.S. Postal employees feed 17 tons of reading matter, labeled by postal authorities as propaganda, into a furnace in San Francisco, California, on March 19, 1941. The bulk of the newspapers, books, and pamphlets came from Nazi Germany and some from Russia, Italy and Japan.

These Arab recruits line up in a barracks square in the British Mandate of Palestine, on December 28, 1940, for their first drill under a British solider. Some 6,000 Palestinian Arabs signed up with the British Army during the course of World War II.

These Arab recruits line up in a barracks square in the British Mandate of Palestine, on December 28, 1940, for their first drill under a British solider. Some 6,000 Palestinian Arabs signed up with the British Army during the course of World War II.

Artillery Signalers at dawn in an outpost in Palestine on December 16. 1940. The men dress warmly to keep out the chill of the desert.

Artillery Signalers at dawn in an outpost in Palestine on December 16. 1940. The men dress warmly to keep out the chill of the desert.

A newly-constructed wall partitions the central part of Warsaw, Poland, seen on December 20, 1940. It is part of red brick and gray stone walls built 12 to 15 feet high by the Nazis as a ghetto - a pen for Warsaw's approximately 500,000 Jews.

A newly-constructed wall partitions the central part of Warsaw, Poland, seen on December 20, 1940. It is part of red brick and gray stone walls built 12 to 15 feet high by the Nazis as a ghetto – a pen for Warsaw’s approximately 500,000 Jews.

A scene from the Warsaw Ghetto where Jews are seen wearing white armlets bearing the Star of David and trams are seen marked with the words

A scene from the Warsaw Ghetto where Jews are seen wearing white armlets bearing the Star of David and trams are seen marked with the words “For Jews Only”, on February 17, 1941.

A German Army officer lecturers children in a ghetto in Lublin, German-occupied Poland, on December 1940, telling them

A German Army officer lecturers children in a ghetto in Lublin, German-occupied Poland, on December 1940, telling them “Don’t forget to wash every day.”

The faces of Jewish children living in a ghetto in Szydlowiec, Poland, under Nazi occupation, on December 20, 1940.

The faces of Jewish children living in a ghetto in Szydlowiec, Poland, under Nazi occupation, on December 20, 1940.

(Photo credit: AP / Library of Congress).