The Russian invasion of Poland began on September 17, 1939, and ended on October 6, 1939, when the Soviet troops had reached a line established by the rivers San, Vistula, and Narew, called the Curzon Line, as they met the German units that had also attacked Poland from the West on September 1. The Soviet assault on Poland from the East had already been agreed on by Hitler and Stalin in the Molotov-Ribbentrop Pact, signed on August 23, 1939. The the tract of land captured by the Russian units constituted about 50% of the Polish territories granted to the Poles by the Treaty of Versailles; it was annexed to the Soviet Union as the Kresy, About 22,000 Polish prisoners of war were mass murdered in Russia by the Soviets in 1940; a brutal episode known as the Katyn Forest Massacre.
The Red Army’s invasion force was made up of 700,000 troops organized in 36 divisions and 12 brigades, under the command of General Kliment Voroshilov. These units were equipped with 4,700 armored vehicles of all sorts, 5,000 guns, and 3,250 aircraft. This invading army attacked Poland in two fronts; on the south, the Ukranian Front, which was the left flank, and on the north, Belarusain Front, which constituted the right flank of the invasion force. On the first day, on September 17, the Soviets defeated the Polish in the Battle of Brzesc and took the Brest Fortress. By September 24, the Red Army had taken the towns of Wilno and Grodno. On October 1, the Red Army defeated a Polish unit at the Battle of Wytyczno. By October 2, Russian spearhead infantry units had reached the Vistula River. Despite the ferocious and stubborn Polish resistance, on October 6, the last Polish Army units surrendered to the Soviet commanders.