Poland and Russia had exchanged diplomatic missions for centuries.
 The first ambassador in the modern meaning of this word, from Poland to Russia, was Antoni Augustyn Deboli, in late 18th century.
 After the period of partitions of Poland, in 1918, relations were established between the Second Polish Republic and Soviet Union.
 After Soviet invasion of Poland in 1939 those relations were broken, to be briefly reestablished in 1941 after the German invasion of the Soviet Union, when the Soviet Union and Polish government in exile agreed to cooperate against their common enemy, Nazi Germany.
 Those relations were broken in 1943 after discovery of the Katyn massacre.
 From that point onward, Soviet Union created its own puppet Polish government, which had its "ambassadors" in the Soviet Union.
 In 1989 the People's Republic of Poland was transformed into the modern Poland; in 1991, Soviet Union was transformed into modern Russia.
The Invasion of Poland, known in Poland as the September Campaign ("Kampania wrześniowa") or the 1939 Defensive War ("Wojna obronna 1939 roku"), and in Germany as the Poland Campaign ("Polenfeldzug") or "Fall Weiss" ("Case White"), was a joint invasion of Poland by Nazi Germany, the Soviet Union, the Free City of Danzig, and a small Slovak contingent, that marked the beginning of World War II.
 The German invasion began on 1 September 1939, one week after the signing of the Molotov–Ribbentrop Pact, while the Soviet invasion commenced on 17 September following the Molotov-Tōgō agreement that terminated the Russian and Japanese hostilities in the east on 16 September.
 The campaign ended on 6 October with Germany and the Soviet Union dividing and annexing the whole of Poland under the terms of the German–Soviet Frontier Treaty.
Poland and Russia had exchanged diplomatic missions for centuries.
 The first ambassador in the modern meaning of this word, from Poland to Russia, was Antoni Augustyn Deboli, in late 18th century.
 After the period of partitions of Poland, in 1918, relations were established between the Second Polish Republic and Soviet Union.
 After Soviet invasion of Poland in 1939 those relations were broken, to be briefly reestablished in 1941 after the German invasion of the Soviet Union, when the Soviet Union and Polish government in exile agreed to cooperate against their common enemy, Nazi Germany.
 Those relations were broken in 1943 after discovery of the Katyn massacre.
 From that point onward, Soviet Union created its own puppet Polish government, which had its "ambassadors" in the Soviet Union.
 In 1989 the People's Republic of Poland was transformed into the modern Poland; in 1991, Soviet Union was transformed into modern Russia.
The Soviet–Japanese Neutrality Pact (日ソ中立条約 , Nisso Chūritsu Jōyaku ) , also known as the Japanese–Soviet Non-aggression Pact (日ソ不可侵条約 , Nisso Fukashin Jōyaku ) , was a pact between the Soviet Union and the Empire of Japan signed on April 13, 1941, two years after the brief Soviet–Japanese Border War (1939).
 The pact was signed to ensure the neutrality between the Soviet Union and the Empire of Japan during World War II, in which both countries participated.
The German–Soviet Credit Agreement (also referred to as the German–Soviet Trade and Credit Agreement) was an economic arrangement between the Soviet Union and Nazi Germany whereby Soviet Union received an acceptance credit of 200 million Reichsmark.
 over 7 years with an effective interest rate of 4.5 percent.
 The credit line was to be used during the next two years for purchase of capital goods (factory equipment, installations, machinery and machine tools, ships, vehicles, and other means of transport) in Germany and was to be paid off by means of Soviet material shipment from 1946 onwards.
 The economic agreement was the first step toward improvement in relations between the Soviet Union and Germany.
 The next day after the Credit Agreement, the Soviet Union went to war against Japan, in a successful four-week military campaign in the Far East.
 The Nazi-Soviet Pact was signed four days after the Credit Agreement.
 The 1939 German–Soviet Commercial Agreement renewed declined Nazi–Soviet economic relations and was adjusted and expanded with the larger German–Soviet Commercial Agreement in February 1940 and January 1941 German–Soviet Border and Commercial Agreement.
 German shipments to the Soviets became tardy and failed to provide all that was promised the closer the date of Barbarossa came.
 The Soviets fulfilled their obligations to the letter right up until the invasion, wanting to avoid provoking Germany.
 All these agreements were terminated when Germany invaded the Soviet Union in June 1941, in violation of the treaties between the two countries.
 Soviet trade with Germany in the pre-invasion period ended up providing the Germans with many of the resources they needed for their invasion of the Soviet Union.
The Molotov–Ribbentrop Pact, also known as the Nazi–Soviet Pact, the German–Soviet Non-aggression Pact or the Nazi German-Soviet Pact of Aggression (officially: "Treaty of Non-aggression between Germany and the Union of Soviet Socialist Republics"), was a neutrality pact between Nazi Germany and the Soviet Union signed in Moscow on 23 August 1939 by foreign ministers Joachim von Ribbentrop and Vyacheslav Molotov, respectively.
 The pact was followed by the German-Soviet Commercial Agreement in February 1940.
The Soviet invasion of Poland was a Soviet military operation that started without a formal declaration of war on 17 September 1939.
 On that morning, 16 days after Germany invaded Poland from the west, the Soviet Union invaded Poland from the east.
 The invasion and the battle lasted for the following 20 days and ended on 6 October 1939 with the two-way division and annexation of the entire territory of the Second Polish Republic by both Germany and the Soviet Union.
 The joint German-Soviet invasion of Poland was secretly agreed to in the Molotov-Ribbentrop Pact, signed on 23 August 1939.
The German-Soviet Frontier Treaty was a second supplementary protocol, of the 1939 Hitler-Stalin Pact (known as the "German-Soviet Treaty of Nonaggression", or by its original name of the "German–Soviet Treaty of Friendship, Cooperation and Demarcation").
 It was a secret clause as amended on September 28, 1939 by Nazi Germany and the Soviet Union after their joint invasion and occupation of sovereign Poland.
 It was signed by Joachim von Ribbentrop and Vyacheslav Molotov, the foreign ministers of Germany and the Soviet Union respectively, in the presence of Joseph Stalin.
 The treaty was a follow-up to the first secret protocol of the Molotov–Ribbentrop Pact signed on August 23, 1939 between the two countries prior to their invasion of Poland and the start of World War II in Europe.
 Only a small portion of the protocol which superseded the first treaty was publicly announced while the spheres of influence of Nazi Germany and the Soviet Union remained classified.
 The third secret protocol of the Pact was signed on January 10, 1941 by Friedrich Werner von Schulenberg, and Molotov, whereas Germany renounced its claims to portions of Lithuania, only a few months before their anti-Soviet Operation Barbarossa.
The Soviet Union signed a non-aggression pact with Nazi Germany on 23 August 1939.
 In addition to stipulations of non-aggression, the treaty included a secret protocol that divided territories of Romania, Poland, Lithuania, Latvia, Estonia and Finland into German and Soviet "spheres of influence", anticipating potential "territorial and political rearrangements" of these countries.
 Stalin and Hitler later traded proposals after a Soviet entry into the Axis Pact.
The Cairo Conference (codenamed Sextant) of November 22–26, 1943, held in Cairo, Egypt, outlined the Allied position against Japan during World War II and made decisions about postwar Asia.
 The meeting was attended by President of the United States Franklin Roosevelt, Prime Minister of the United Kingdom Winston Churchill, and Generalissimo Chiang Kai-shek of the Republic of China.
 Soviet leader Joseph Stalin did not attend the conference because his meeting with Chiang could have caused friction between the Soviet Union and Japan.
 (The Soviet-Japanese Neutrality Pact of 1941 was a five-year agreement of neutrality between the two nations; in 1943 the Soviet Union was not at war with Japan, whereas China, the U.K. and the U.S. were.)

Question: The German invasion began one week after the signing of what neutrality pact between Nazi Germany and the Soviet Union signed in Moscow on 23 August 1939?
Answer: Molotov–Ribbentrop Pact