On 17 September 1939, sixteen days after the start of World War II and with the victorious Germans having advanced deep into Polish territory, the Red Army invaded eastern Poland, stating as justification the "need to protect Ukrainians and Belarusians" there, after the "cessation of existence" of the Polish state. As a result, the Belarusian and Ukrainian Soviet republics' western borders were moved westward, and the new Soviet western border was drawn close to the original Curzon line. In the meantime negotiations with Finland over a Soviet-proposed land swap that would redraw the Soviet-Finnish border further away from Leningrad failed, and in December 1939 the USSR invaded Finland, beginning a campaign known as the Winter War (1939–40). The war took a heavy death toll on the Red Army but forced Finland to sign a Moscow Peace Treaty and cede the Karelian Isthmus and Ladoga Karelia. In summer 1940 the USSR issued an ultimatum to Romania forcing it to cede the territories of Bessarabia and Northern Bukovina. At the same time, the Soviet Union also occupied the three formerly independent Baltic states (Estonia, Latvia and Lithuania).