The Soviet invasion of Poland was a Soviet military operation that started without a formal declaration of war on 17 September 1939 , immediately after the end of the undeclared war between the Soviet Union and the Empire of Japan at the Battles of Khalkhin Gol ( Nomonhan ) in the Far East .
The 1939 Soviet invasion of Poland was a Soviet military operation that started without a formal declaration of war on 17 September 1939 .