In what war did the Soviet Union fight the Kwantung Army?

The Kwantung Army was the largest and best equipped army group of the Japanese Imperial Army during the Second World War. It was defeated in 1945 by the Soviet Red Army during the Soviet-Japanese War, a part of the larger global conflict, World War II.

The Soviet Army invaded Manchuria in August 1945 as part of the agreement made during the Yalta Conference for the Soviet Union to enter into the Pacific war following the defeat of Nazi Germany. The three week offensive lead to the defeat of the Kwantung army and ended in Manchuria and northern Korea under Soviet control.

Soviet and Japanese forces had fought earlier, during the 1930s, in a series of border skirmishes and battles that lead to the signing of the Soviet-Japanese Neutrality Pact which kept peace between Japan and the Soviet Union until August 1945. The neutrality pact forced Japan to focus its expansion to Southeast Asia and the Pacific.

The other conflicts listed, the Russian Civil War occurred after the end of WWI, the Russo-Japanese war occurred between the Russian Empire and Japan in 1904-1905, Japan defeated Russia, marking Japan's rise to the world stage as a power. The First World War, Russia and Japan were on the same side.