Complex sentence: The Nazi-Soviet Pact signed in secrecy led to the parallel Soviet invasion of Poland from the east on September 17,1939 and the forcible annexation of eastern half of prewar Poland into the Soviet Belarus and the Soviet Ukraine in the atmosphere of terror.
Split into simpler sentences: The Nazi-Soviet Pact signed in secrecy led to the parallel Soviet invasion of Poland from the east on September 17,1939. The eastern half of prewar Poland was annexed by the USSR to the two republics of Soviet Belarus and Soviet Ukraine in the atmosphere of terror.