Peace of Riga

The Peace of Riga, or The Treaty of Riga (Polish:"Traktat Ryski)", was signed in Riga on March 18 1921 by the Second Polish Republic, Soviet Russia, and Soviet Ukraine. The treaty ended the Polish-Soviet War.
The borders between Poland and Russia that were changed by the treaty stayed the same until World War II. They were then changed by the Yalta Conference and the Potsdam Conference.
Background.
WW1.
World War I changed the imperial borders of Europe. In 1918, after the Russian Revolution, Russia had given up Poland in the Treaty of Brest-Litovsk and the war ended when Germany surrendered. Poland was re-created after over 100 years of being split up by three countries.
Russian Civil War.
The Russian Civil War gave Poland the chance to regain the land controlled by the Polish-Lithuanian Commonwealth that had been taken by Russia in the 18th century. Meanwhile, the Soviet leaders wanted to respond to the leader of Poland, Józef Piłsudski, who had moved into Ukraine, by attacking Poland, which the Soviets saw as a land bridge to Western Europe, which they could cross to expand communism to the west. That started the Polish-Soviet War and ended in a victory for Poland after the Battle of Warsaw, which made both sides tired and want to end the war. Other victories by the Poles placed them under pressure by the League of Nations, which also wanted to end the war after they had taken a lot of land.
Negotiations.
Peace talks started in Minsk on August 17, 1920, but since the Polish counterattack had moved closer, they moved to Riga and continued on September 21. The Soviets suggested two solutions, one on 21 September and a second on 28 September. The Poles suggested another solution on October 2. Three days later, the Soviets suggested a change to the offer to the Poles, who accepted. An end to the war was signed on 12 October, and the war actually ended on 18 October 1920. The leading negotiators were, Jan Dąbski from Poland and Adolph Joffe from Soviet Russia. The Soviets did not let the noncommunist Ukrainian leaders be in the negotiations. 
The Soviets' military problems made them give up lots of the land that they wanted. However to many other people, it looked as if Poland had lost the war. Most of the Polish negotiators were members of the National Democrats, who were Piłsudski's political enemies. They did not want more than a third of the people living in the bigger Poland to be non-Polish and so they agreed to get less land than they would have gotten even though many Poles would be left on the Soviet side of the border, and the Soviets had been willing to give up even Minsk