View of an undamaged Polish city from the cockpit of a German medium bomber aircraft, likely a Heinkel He 111 P, in 1939.

View of an undamaged Polish city from the cockpit of a German medium bomber aircraft, likely a Heinkel He 111 P, in 1939.

On September 1, 1939, the world was shocked as Nazi troops invaded Poland. Tensions soared in Europe, and world leaders recognized that this invasion could be the final straw that would lead to war. German military leaders had begun planning for war with Poland as early as the mid 1920s. Recovering the ethnically Polish territory of Pomerania, Poznan, and Silesia, as well as the largely German Free City of Danzig were the major objectives. Nevertheless, the restrictions of Versailles and Germany’s internal weakness made such plans impossible to realize.

Hitler’s rise to power in 1933 capitalized on German’s desire to regain lost territories, to which Nazi leaders added the goal of destroying an independent Poland. According to author Alexander Rossino, prior to the war Hitler was at least as anti-Polish as anti-Semitic in his opinions.

After Hitler violated the Munich treaty, Poland was able to extract guarantees of military assistance from France, and significantly, Britain. In March 1939, Hitler began to make demands on Poland for the return of territory in the Polish Corridor, cessation of Polish rights in Danzig, and annexation of the Free City to Germany. These Poland categorically rejected. The war was close.

In 1939, the Polish army still maintained many cavalry squadrons, which had served them well as recently as the Polish-Soviet War in 1921. A myth emerged about the Polish cavalry leading desperate charges against the tanks of the invading Nazis, pitting horsemen against armored vehicles. While cavalry units did encounter armored divisions on occasion, their targets were ground infantry, and their charges were often effective. Nazi and Soviet propaganda helped fuel the myth of the noble-yet-backward Polish cavalry. This photo is of a Polish cavalry squadron on maneuvers somewhere in Poland, on April 29, 1939.

In 1939, the Polish army still maintained many cavalry squadrons, which had served them well as recently as the Polish-Soviet War in 1921. A myth emerged about the Polish cavalry leading desperate charges against the tanks of the invading Nazis, pitting horsemen against armored vehicles. While cavalry units did encounter armored divisions on occasion, their targets were ground infantry, and their charges were often effective. Nazi and Soviet propaganda helped fuel the myth of the noble-yet-backward Polish cavalry. This photo is of a Polish cavalry squadron on maneuvers somewhere in Poland, on April 29, 1939.

On paper, Poland’s full mobilized army would have numbered about 2.5 million. Due to allied pressure and mismanagement, however, only about 600,000 Polish troops were in place to meet the German invasion on September 1, 1939. These forces were organized into 7 armies and 5 independent operational groups. The typical Polish infantry division was roughly equal in numbers to its German counterpart, but weaker in terms of anti-tank guns, artillery support, and transport. Poland had 30 active and 7 reserve divisions. In addition there were 12 cavalry brigades and one mechanized cavalry brigade.

The Germans were organized in two Army Groups, with a total of 5 armies. The Germans fielded about 1.8 million troops. The Germans had 2600 tanks against the Polish 180, and over 2,000 aircraft against the Polish 420. German forces were supplemented by a Slovak brigade.

German forces invaded Poland from the north, south, and west the morning after the Gleiwitz incident. Slovak forces advanced alongside the Germans in northern Slovakia. As the Wehrmacht advanced, Polish forces withdrew from their forward bases of operation close to the Polish–German border to more established lines of defence to the east. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage. Polish forces then withdrew to the southeast where they prepared for a long defence of the Romanian Bridgehead and awaited expected support and relief from France and the United Kingdom. While those two countries had pacts with Poland and had declared war on Germany on 3 September, in the end their aid to Poland was very limited.

The Soviet Red Army’s invasion of Eastern Poland on 17 September, in accordance with a secret protocol of the Molotov–Ribbentrop Pact, rendered the Polish plan of defence obsolete. Facing a second front, the Polish government concluded the defence of the Romanian Bridgehead was no longer feasible and ordered an emergency evacuation of all troops to neutral Romania. On 6 October, following the Polish defeat at the Battle of Kock, German and Soviet forces gained full control over Poland. The success of the invasion marked the end of the Second Polish Republic, though Poland never formally surrendered.

Associated Press correspondent Alvin Steinkopf broadcasting from the Free City of Danzig -- at the time, a semi-autonomous city-state tied to Poland. Steinkopf was relating the tense situation in Danzig back to America, on July 11, 1939. Germany had been demanding the incorporation of Danzing into the Third Reich for months, and appeared to be preparing military action.

Associated Press correspondent Alvin Steinkopf broadcasting from the Free City of Danzig — at the time, a semi-autonomous city-state tied to Poland. Steinkopf was relating the tense situation in Danzig back to America, on July 11, 1939. Germany had been demanding the incorporation of Danzing into the Third Reich for months, and appeared to be preparing military action.

The American journalist John Gunther wrote in December 1939 that “the German campaign was a masterpiece. Nothing quite like it has been seen in military history”. Despite Poland’s poor leadership and outside assistance, Gunther still claimed that the invasion proved the skill of the German armed forces. The country was divided between Germany and the Soviet Union. Slovakia gained back those territories taken by Poland in autumn 1938. Lithuania received the city of Vilnius and its environs on 28 October 1939 from the Soviet Union.

About 65,000 Polish troops were killed in the fighting, with 420,000 others being captured by the Germans and 240,000 more by the Soviets (for a total of 660,000 prisoners). Up to 120,000 Polish troops escaped to neutral Romania(through the Romanian Bridgehead and Hungary), and another 20,000 to Latvia and Lithuania, with the majority eventually making their way to France or Britain. Most of the Polish Navy succeeded in evacuating to Britain as well. German personnel losses were less than their enemies (c. 16,000 killed).

Soviet premier Josef Stalin (second from right), smiles while Soviet Foreign Minister Vyacheslav Molotov (seated), signs the non-aggression pact with German Reich Foreign Minister Joachim von Ribbentrop (third from right), in Moscow, on August 23, 1939. The man at left is Soviet Deputy Defense Minister and Chief of the General Staff, Marshal Boris Shaposhnikov. The nonaggression pact included a secret protocol dividing eastern Europe into spheres of influence in the event of a conflict. The pact now guaranteed that Hitler's troops would face no resistance from the Soviets if they invaded Poland, bringing the war one step closer to reality.

Soviet premier Josef Stalin (second from right), smiles while Soviet Foreign Minister Vyacheslav Molotov (seated), signs the non-aggression pact with German Reich Foreign Minister Joachim von Ribbentrop (third from right), in Moscow, on August 23, 1939. The man at left is Soviet Deputy Defense Minister and Chief of the General Staff, Marshal Boris Shaposhnikov. The nonaggression pact included a secret protocol dividing eastern Europe into spheres of influence in the event of a conflict. The pact now guaranteed that Hitler’s troops would face no resistance from the Soviets if they invaded Poland, bringing the war one step closer to reality.

Two days after Germany signed the non-aggression pact with the USSR, Great Britain entered into a military alliance with Poland, on August 25, 1939. This photo shows the scene one week later, on September 1, 1939, one of the first military operations of Germany's invasion of Poland, and the beginning of World War II. Here, the German battleship Schleswig-Holstein is bombing a Polish military transit depot at Westerplatte in the Free City of Danzig. Simultaneously, the German Air Force (Luftwaffe), and ground troops (Heer) were attacking several other Polish targets.

Two days after Germany signed the non-aggression pact with the USSR, Great Britain entered into a military alliance with Poland, on August 25, 1939. This photo shows the scene one week later, on September 1, 1939, one of the first military operations of Germany’s invasion of Poland, and the beginning of World War II. Here, the German battleship Schleswig-Holstein is bombing a Polish military transit depot at Westerplatte in the Free City of Danzig. Simultaneously, the German Air Force (Luftwaffe), and ground troops (Heer) were attacking several other Polish targets.

German soldiers comb the Westerplatte after it was surrendered to German units from the Schleswig-Holstein landing crew, on September 7, 1939. Fewer than 200 Polish soldiers defended the small peninsula, holding off the Germans for seven days.

German soldiers comb the Westerplatte after it was surrendered to German units from the Schleswig-Holstein landing crew, on September 7, 1939. Fewer than 200 Polish soldiers defended the small peninsula, holding off the Germans for seven days.

Aerial view of bombs exploding during a German bombing run over Poland in September of 1939.

Aerial view of bombs exploding during a German bombing run over Poland in September of 1939.

Two tanks of the SS-Leibstandarte Adolf Hitler Division cross the Bzura River during the German invasion of Poland in September of 1939. The Battle of Bzura, the largest of the entire campaign, lasted more than a week, ending with the German forces capturing most of western Poland.

Two tanks of the SS-Leibstandarte Adolf Hitler Division cross the Bzura River during the German invasion of Poland in September of 1939. The Battle of Bzura, the largest of the entire campaign, lasted more than a week, ending with the German forces capturing most of western Poland.

Soldiers of the SS-Leibstandarte Adolf Hitler Division, resting in a ditch alongside a road on the way to Pabianice, during the invasion of Poland in 1939.

Soldiers of the SS-Leibstandarte Adolf Hitler Division, resting in a ditch alongside a road on the way to Pabianice, during the invasion of Poland in 1939.

German advance guards and scouts are shown in a Polish town that has been under fire during the Nazi invasion of Poland, September 1939.

German advance guards and scouts are shown in a Polish town that has been under fire during the Nazi invasion of Poland, September 1939.

German infantry cautiously advance on the outskirts of Warsaw, Poland on September 16, 1939.

German infantry cautiously advance on the outskirts of Warsaw, Poland on September 16, 1939.

Several civilian prisoners of war, with arms raised, walk along a road during the German invasion of Poland in September of 1939.

Several civilian prisoners of war, with arms raised, walk along a road during the German invasion of Poland in September of 1939.

Britain's King George VI broadcasts to the British nation on the first evening of the war, on September 3, 1939, in London.

Britain’s King George VI broadcasts to the British nation on the first evening of the war, on September 3, 1939, in London.

A crowd reads newspaper headlines,

A crowd reads newspaper headlines, “Bombs Rain On Warsaw” as they stand outside the U.S. State Department building where diplomats held a conference on war conditions in Europe, on September 1, 1939.

The scene of devastation seen on Ordynacka Street in Warsaw, Poland on March 6, 1940. The carcass of a dead horse lies in the street among enormous piles of debris. While Warsaw was under nearly constant bombardment during the invasion, on one day alone, September 25, 1939, about 1,150 bombing sorties were flown by German aircraft against Warsaw, dropping over 550 tons of high explosive and incendiary bombs on the city.

The scene of devastation seen on Ordynacka Street in Warsaw, Poland on March 6, 1940. The carcass of a dead horse lies in the street among enormous piles of debris. While Warsaw was under nearly constant bombardment during the invasion, on one day alone, September 25, 1939, about 1,150 bombing sorties were flown by German aircraft against Warsaw, dropping over 550 tons of high explosive and incendiary bombs on the city.

A damaged Polish armored train carrying tanks captured by the 14th SS-Leibstandard Adolf Hitler Division, near Blonie, during the invasion of Poland in September of 1939.

A damaged Polish armored train carrying tanks captured by the 14th SS-Leibstandard Adolf Hitler Division, near Blonie, during the invasion of Poland in September of 1939.

German soldiers, taken prisoner by the Polish army during the Nazi invasion, are shown while they were held captive in Warsaw, on October 2, 1939.

German soldiers, taken prisoner by the Polish army during the Nazi invasion, are shown while they were held captive in Warsaw, on October 2, 1939.

A young Polish boy returns to what was his home and squats among the ruins during a pause in the German air raids on Warsaw, Poland, in September of 1939. German attacks lasted until Warsaw surrendered on September 28. One week later, the last of the Polish forces capitulated near Lublin, giving full control of Poland to Germany and the Soviet Union.

A young Polish boy returns to what was his home and squats among the ruins during a pause in the German air raids on Warsaw, Poland, in September of 1939. German attacks lasted until Warsaw surrendered on September 28. One week later, the last of the Polish forces capitulated near Lublin, giving full control of Poland to Germany and the Soviet Union.

Adolf Hitler salutes parading troops of the German Wehrmacht in Warsaw, Poland, on October 5, 1939 after the German invasion. Behind Hitler are, from left to right: Colonel General Walther von Brauchitsch, Lieutenant General Friedrich von Cochenhausen, Colonel General Gerd von Rundstedt, and Colonel General Wilhelm Keitel.

Adolf Hitler salutes parading troops of the German Wehrmacht in Warsaw, Poland, on October 5, 1939 after the German invasion. Behind Hitler are, from left to right: Colonel General Walther von Brauchitsch, Lieutenant General Friedrich von Cochenhausen, Colonel General Gerd von Rundstedt, and Colonel General Wilhelm Keitel.

(Photo credit: Bundesarchiv / AP).