Some Polish contributions were less visible, notably the prewar and wartime decrypting of German Enigma-machine ciphers by cryptologists Marian Rejewski, Henryk Zygalski, and Jerzy Różycki.

On 6 October, following the Polish defeat at the Battle of Kock, German and Soviet forces gained full control over Poland.

The Polish forces in the West, as well as in the East and an intelligence service were established outside of Poland, and contributed to the Allied effort throughout the war.

The invasion of Polish Second Republic by the military forces of Nazi Germany marked the beginning of World War II in Europe.

[10] On September 17 the Soviets invaded eastern Poland, forcing the Polish government and military to abandon their plans for a long-term defense in the Romanian bridgehead area.

The campaign began on September 1, 1939, one week after the signing of the Molotov–Ribbentrop Pact containing a secret protocol for the division of Northern and Central Europe into German and Soviet spheres of influence.

German-occupied Poland was the only European territory where the Germans punished any kind of help to Jews with death for the helper and his entire family.

[13] Of the individuals awarded medals of Righteous among the Nations (given by the State of Israel to non-Jews who saved Jews from extermination in the Holocaust) those who were Polish citizens number the greatest.

[14] There are 6,339[15] Polish men and women recognized as "Righteous" to this day, amounting to over 25 percent of the total number of 22,765 honorary titles awarded already.

[16] The main resistance force in German-occupied Poland was the Armia Krajowa ("Home Army"; abbreviated "AK"), which numbered some 400,000 fighters at its peak as well as many more sympathizers.

[b] The AK coordinated its operations with the exiled Polish Government in London and its activity concentrated on sabotage, diversion and intelligence gathering.

[18] Before that, AK units carried out thousands of raids, intelligence operations, bombed hundreds of railway shipments, participated in many clashes and battles with the German police and Wehrmacht units and conducted tens of thousands of acts of sabotage against German industry[20] The AK also conducted "punitive" operations to assassinate Gestapo officials responsible for Nazi terror.

From 1943 onwards, some units took part in battling the Gwardia Ludowa and the Polish People's Army PAL, both communist resistance movement.

As of July, 1944 it incorporated a similar organization, the Gwardia Ludowa and the Polish People's Army PAL, and numbered about 6,000 soldiers (although estimates vary).

[27] Despite Poland becoming occupied, the Polish intelligence network not only survived but grew rapidly, and near the end of the war had over 1,600 registered agents[26] (Another estimate gave about 3,500[citation needed]).

[28][29][30] According to Marek Ney-Krwawicz [pl], for the Western Allies, the intelligence provided by the Home Army was considered to be the best source of information on the Eastern Front.

[31] In a period of more than six and a half years, from late December 1932 to the outbreak of World War II, three mathematician-cryptologists (Marian Rejewski, Henryk Zygalski and Jerzy Różycki) at the Polish General Staff's Cipher Bureau in Warsaw had developed a number of techniques and devices— including the "grill" method, Różycki's "clock", Rejewski's "cyclometer" and "card catalog", Zygalski's "perforated sheets", and Rejewski's "cryptologic bomb" (in Polish, "bomba", precursor to the later British "Bombe", named after its Polish predecessor)— to facilitate decryption of messages produced on the German "Enigma" cipher machine.

Just five weeks before the outbreak of World War II, on July 25, 1939, near Pyry in the Kabaty Woods south of Warsaw, Poland disclosed her achievements to France and the United Kingdom, which had, up to that time, failed in all their own efforts to crack the German military Enigma cipher.

[30] In July 1941 Mieczysław Słowikowski (codenamed "Rygor"—Polish for "Rigor") set up "Agency Africa", one of World War II's most successful intelligence organizations.

[32] The information gathered by the Agency was used by the Americans and British in planning the amphibious November 1942 Operation Torch[39][better source needed] landings in North Africa.

[citation needed] Some Poles also served in other Allied intelligence services, including the celebrated Krystyna Skarbek ("Christine Granville") in the United Kingdom's Special Operations Executive.

It took part in battles for Bydgoszcz (Bromberg), Kolobrzeg (Kolberg), Gdańsk (Danzig) and Gdynia, losing about 17,500 killed in action over the course of the war.

Hundreds of thousands of former Polish citizens, particularly residents of parts of Poland annexed to Germany, were conscripted into the German Armed Forces.

The Polish Home Army was probably the only World War II resistance movement to manufacture large quantities of weaponry and munitions.

The KIS was designed and made in the Jan Piwnik's "Ponury" ("Grim") guerrilla unit that was operating in Holy Cross Mountains region.

[citation needed] a ^ Numerous sources state that Polish Army was the fourth biggest Allied fighting contingent.

Steven J. Zaloga wrote that "by the war's end the Polish Army was the fourth largest contingent of the Allied coalition after the armed forces of the Soviet Union, the United States and Great Britain.

"[69] Jerzy Jan Lerski writes "All in all, the Polish units, although divided and controlled by different political orientation, constituted the fourth largest Allied force, after the American, British and Soviet Armies.

"[70] M. K. Dziewanowski has noted that "if Polish forces fighting in the east and west were added to the resistance fighters, Poland had the fourth largest Allied army in the war (after the USSR, the U.S. and Britain)".

Polish territories were mostly freed from Nazi German control in the years 1944–1945, eliminating the need for their respective (anti-Nazi) partisan forces in Poland (although the cursed soldiers continued to fight against the Soviets).