On 1 September, barely a week after the pact had been signed, Germany invaded Poland. The Soviet Union invaded Poland from the east on 17 September and on 28 September signed a secret treaty with Nazi Germany to coordinate fighting against the Polish resistance. The Soviets targeted intelligence, entrepreneurs and officers, committing a string of atrocities that culminated in the Katyn massacre and mass relocation to the Gulag in Siberia. Soon after the invasion of Poland, the Soviet Union occupied the Baltic countries of Estonia, Latvia and Lithuania, and annexed Bessarabia and Northern Bukovina from Romania. The Soviet Union attacked Finland on 30 November 1939, which started the Winter War. Finnish defenses prevented an all-out invasion, resulting in an interim peace, but Finland was forced to cede strategically important border areas near Leningrad.