By the treaty terms signed on that day, the Soviet Union acquired the Karelian territories and the port of Viborg, as well as other strategic and economic advantages.

Expansion in the Baltic and the Balkans

Soviet expansion continued during 1940. On June 15-16 the USSR demanded free passage of Soviet troops and the formation of pro-Soviet governments in Latvia, Lithuania, and Estonia. Without waiting for acceptance of these demands, the Red Army occupied the countries. Soviet puppet governments were established, and all anti-Soviet elements were suppressed. By decrees of the Supreme Soviet of the USSR, issued between August 1 and August 8, the three states were annexed as Union republics.

At the same time the Soviet Union was extending its reach to the Balkans. Demands were made on Romania for the cession of Bessarabia, annexed by Romania from Soviet Russia in 1918, and for the surrender of northern Bukovina. Romania complied at the end of June 1940; the ceded territories were later incorporated into the Moldavian SSR. In the fall of 1940 the Germans established a puppet government in Romania and guaranteed the Romanian-Soviet frontier.

Still fearful of German intentions, the USSR had an interest in ending hostilities with Japan; on April 13, 1941, the two countries signed a five-year neutrality pact.

German invasion

On June 22, 1941, Germany invaded the Soviet Union, surprising Stalin, who had refused to believe that an attack was imminent. Italy and Romania declared war on the USSR the same day. Instantly the world military and political alignment was radically transformed, and the scope of the war began to assume global proportions. Germany now confronted enemies on both west and east, as in World War I. As Finland, Hungary, Albania, and other Axis satellites declared war on the Soviet Union, Britain and the United States undertook to extend material aid to the USSR. The US programme, known as Lend-Lease, ultimately provided the USSR with some $12 billion worth of equipment and food. After the United States entered the war in December 1941, the three powers became military allies. In January 1942, four months after it had pledged allegiance to the principles of the Atlantic Charter, the Soviet government and 25 other Allied governments signed a declaration formally subscribing to the programme and purposes of the Atlantic Charter and pledging their cooperation in the war against the Axis powers.

The Axis assault on the USSR was launched from the Arctic Ocean to the Black Sea. During the late summer and fall of 1941 the Germans plunged deeply into the Soviet Union, striking at Leningrad, Moscow, and the Ukraine.