World War II: Germany and the Soviet Union sign a non-aggression treaty, the Molotov-Ribbentrop Pact. In a secret addition to the pact, the Baltic states, Finland, Romania, and Poland are divided between the two nations.

World War II: The Soviet Union joins Nazi Germany's invasion of Poland during the Polish Defensive War of 1939.

Nazi Germany and the Soviet Union agree on a division of Poland after their invasion during World War II.

Winter War: The Soviet Union is expelled from the League of Nations for invading Finland.

Winter War: Finland signs the Moscow Peace Treaty with the Soviet Union, ceding almost all of Finnish Karelia. Finnish troops and the remaining population are immediately evacuated.

The Yermolayev Yer-2, a long-range Soviet medium bomber, has its first flight.

The Soviet Union presents an ultimatum to Lithuania resulting in Lithuanian loss of independence.

The three Baltic states of Estonia, Latvia and Lithuania fall under the occupation of the Soviet Union.

World War II: under the Molotov-Ribbentrop Pact, the Soviet Union presents an ultimatum to Romania requiring it to cede Bessarabia and the northern part of Bukovina.

Romania cedes Bessarabia (current-day Moldova) to the Soviet Union.

The United States' Under Secretary of State Sumner Welles issues a declaration on the U.S. non-recognition policy of the Soviet annexation and incorporation of three Baltic States: Estonia, Latvia and Lithuania.

World War II: the Soviet Union formally annexes Latvia.

Pact of neutrality between the USSR and Japan is signed.

June deportation, the first major wave of Soviet mass deportations and murder of Estonians, Latvians and Lithuanians, begins.

Germany invades the Soviet Union in Operation Barbarossa. The Soviet 22 June Song is dedicated to this day.

The Lithuanian Activist Front declares independence from the Soviet Union and forms the Provisional Government of Lithuania; it lasts only briefly as the Nazis will occupy Lithuania a few weeks later.

Soviet leader Joseph Stalin consolidates the Commissariats of Home Affairs and National Security to form the NKVD and names Lavrenti Beria its chief.

World War II: Siege of Leningrad begins. German forces begin a siege against the Soviet Union's second-largest city, Leningrad.

World War II: concerned that Reza Pahlavi the Shah of Persia is about to ally his petroleum-rich empire with Nazi Germany during World War II, the United Kingdom and the Soviet Union invade Iran in late August and force the Shah to abdicate in favor of his son, Mohammad Reza Pahlavi.

World War II: In their invasion of the Soviet Union, Germany reaches the Sea of Azov with the capture of Mariupol.

World War II: Field Marshal Georgy Zhukov takes command of Red Army operations to prevent the further advance into Russia of German forces and to prevent the Wehrmacht from capturing Moscow.

World War II: Temperatures around Moscow drop to -12 ° C and the Soviet Union launches ski troops for the first time against the freezing German forces near the city.

World War II: The United Kingdom declares war on Finland in support of the Soviet Union during the Continuation War.

Diplomatic relations between the Netherlands and the Soviet Union are established.

World War II: Soviet leader Joseph Stalin issues Order No. 227 in response to alarming German advances into the Soviet Union. Under the order all those who retreat or otherwise leave their positions without orders to do so were to be immediately executed.

The Soviet Union establishes diplomatic relations with Australia.

The Soviet Union begins the forced deportation of the Chechen and Ingush people from the North Caucasus to Central Asia.

Deportation of Crimean Tatars by the Soviet Union government.

World War II: the Soviet Union invades East Karelia and the previously Finnish part of Karelia, occupied by Finland since 1941.

World War II: Romania is captured by the Soviet Union.

World War II: The Fatherland Front takes power in Bulgaria through a military coup in the capital and armed rebellion in the country. A new pro-Soviet government is established.

Armistice between Finland and Soviet Union is signed. (End of the Continuation War).

Soviet Union invades Czechoslovakia.

The Soviet Union recognizes the new pro-Soviet government of Poland.

Soviet forces capture the almost completely destroyed Polish city of Warsaw.

World War II: The ''Wilhelm Gustloff'', overfilled with refugees, sinks in the Baltic Sea after being torpedoed by a Soviet submarine, leading to the deadliest known maritime disaster, killing approximately 9,000 people.

World War II: Capitulation of German garrison in Poznań. The city is liberated by Soviet and Polish forces.

World War II: Soviet Union forces invade Austria and take Vienna; Polish and Soviet forces liberate Gdańsk.

Diplomatic relations between the Soviet Union and Brazil are established.

Cold War: Yugoslav leader Josip "Tito" Broz signs an agreement with the Soviet Union to allow "temporary entry of Soviet troops into Yugoslav territory".