The Invasion of Poland in pictures, 1939

On September 1, 1939, Nazi troops stunned the world when they invaded Poland. Tensions rose in Europe, and world leaders recognized that the invasion could be the last straw that would lead to war. German military leaders began planning war with Poland in the mid-1920s.

Recovering the ethnically Polish territory of Pomerania, Pozna and Silesia, as well as the largely German free city of Danzig, were major objectives. Nevertheless, the sanctions at Versailles and the internal weakness of Germany made such plans impossible to realize.

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

After Hitler violated the Munich Treaty, Poland was able to guarantee military aid from France, and importantly, Britain. In March 1939, Hitler began demanding the return of territory to the Polish Corridor over Poland, the termination of Polish rights in Danzig, and the annexation of the Free City to Germany. This Poland categorically rejected. The war was near.

On paper, Poland's fully mobilized army would have numbered around 2.5 million. However, due to Allied pressure and mismanagement, only about 600,000 Polish troops were present to face the German offensive on September 1, 1939. These forces were organized into 7 armies and 5 independent operational groups.

The typical Polish infantry division was comparable in numbers to its German counterpart but was 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 Brigade and one Mechanized Cavalry Brigade.

The Germans were organized into two army groups with a total of 5 armies. The Germans fielded about 1.8 million soldiers. The Germans had 2600 tanks against the Polish 180, and over 2,000 aircraft against the Polish 420. The German army was supplemented by a Slovak brigade.

The morning after the Glewitz incident, German forces invaded Poland from the north, south and west. The Slovak army advanced with the Germans in northern Slovakia. As the Wehrmacht advanced, Polish forces withdrew from their forward bases of operation near the Polish–German border to more established lines of defense in the east.

After the Polish defeat at the Battle of Bazura in mid-September, the Germans gained an undeniable advantage. Polish forces withdrew to the southeast where they prepared for a long defense of the Romanian bridgehead and awaited expected support and relief from France and the United Kingdom.

While both of those countries had made a pact with Poland and 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 defense obsolete.

Facing a second front, the Polish government concluded that the defense of the Romanian bridgehead was no longer possible and ordered an emergency evacuation of all troops to neutral Romania.

On 6 October, following the Polish defeat at the Battle of Kok, German and Soviet forces gained complete control of Poland. The success of the invasion marked the end of the Second Polish Republic, although Poland never formally surrendered.

American journalist John Gunther wrote in December 1939 that "the German campaign was a masterpiece. Nothing like this has been seen in military history." Despite Poland's poor leadership and outside aid, Günther still claimed that the invasion proved the prowess of the German armed forces.

The country was divided between Germany and the Soviet Union. Slovakia regained those territories occupied by Poland in the autumn of 1938. Lithuania received the city of Vilnius and its environs from the Soviet Union on 28 October 1939.

About 65,000 Polish soldiers were killed in the fighting, another 420,000 were captured by the Germans and over 240,000 by the Soviets (for a total of 660,000 prisoners).

By 120,000 Polish troops fled to neutral Romania (via the Romanian bridgehead and Hungary) and over 20,000 to Latvia and Lithuania, with the majority, eventually making their way to France or Britain. Most of the Polish navy was successful in evacuating Britain as well. German personnel losses were less than that of their enemies (c. 16,000 killed).