Poland invaded Russia in April, after demanded a return to the boundries of 1772. The Poles were intially very succesful capturing Kiev and much of the Ukraine. The Soviets counterattacked, driving off the Poles and advanced towards Warsaw. The Poles however received vigorous aid from the French and were able to repulse the Soviets Under the Treaty of Riga the final frontiers between Poland and the Soviet Union were set.
The Russian Civil War provided the newly independent Polish State an opportunity to expand its borders Eastward. Their hope was an expanded Poland would provide a barrier for future Russian expansion. The Poles were able to defeat a newly established Ukraine's army's forces and thus initially gain control of most of Western Ukraine. However, in May 1920, the Soviet Army launched a counteroffensive. They saw the defeat of the Poles as a means of spreading the Russian Revolution westward toward Germany.
The Red Army advanced all the way to Warsaw as they approached Warsaw, the Polish Army facing what seemed like almost an inevitable defeat counterattacked on August 12, 1920. The attack led by Polish General Jozef Pilsudski was victorious, defeating the Soviet forces and forcing them to retreat. The Soviets lost 10,000 killed, 30,000 wounded, and 50,000 Soviet troops were taken, prisoner.
The Poles continued their attack on Soviet troops who were forced to withdraw further westward. The continued advance of the Polish army forced the Soviets to sue for peace. A ceasefire went into effect on October 18, 1920. A peace treaty titled the Peace of Riga was signed on March 18, 1921. Under the terms of the treaty, the Polish border has moved eastward 200 miles. This became the Polish border until World War II.