The Polish offensive was met by a successful Red Army counterattack. The Soviet operation pushed the Polish forces back westward all the way to the Polish capital, Warsaw, while the Directorate of Ukraine fled to Western Europe. Western fears of Soviet troops arriving at the German frontiers increased the interest of Western powers in the war. In midsummer, the fall of Warsaw seemed certain but in mid-August, the tide had turned again, as the Polish forces achieved an unexpected and decisive victory at the Battle of Warsaw. In the wake of the Polish advance eastward, the Soviets sued for peace and the war ended with a ceasefire in October 1920.
The Peace of Riga was signed on 18 March 1921, dividing the disputed territories between Poland and Soviet Russia. The war largely determined the Soviet–Polish border for the interbellum. Poland gained a territory of around 200 kilometers east of its former border, the Curzon Line which had been defined by an international commission after World War I. Much of the territory allocated to Poland in the Treaty of Riga became part of the Soviet Union after World War II, when the common border was re-defined by the Allied Powers in close accordance with the Curzon Line.
The war is known by several names. "Polish–Soviet War" is the most common but other names include "Russo–Polish War [or Polish–Russian War] of 1919–1921"[N 2] (to distinguish it from earlier Polish–Russian wars) and "Polish–Bolshevik War". This second term (or just "Bolshevik War" (Polish: Wojna bolszewicka)) is most common in Polish sources. In some Polish sources it is also referred as the "War of 1920" (Polish: Wojna 1920 roku).[N 3]
There is disagreement over the dates of the war. The Encyclopædia Britannica begins its article with the date range 1919–1920 but then states, "Although there had been hostilities between the two countries during 1919, the conflict began when the Polish head of state Józef Pilsudski formed an alliance with the Ukrainian nationalist leader Symon Petlyura (21 April 1920) and their combined forces began to overrun Ukraine, occupying Kiev on 7 May."[N 2] The Polish Internetowa encyklopedia PWN, as well as Western historians such as Norman Davies, consider 1919 the starting year of the war.
The ending date is given as either 1920 or 1921; this confusion stems from the fact that while the ceasefire was put in force in the autumn of 1920, the official treaty ending the war was signed months later, in March 1921. While the events of 1919 can be described as a border conflict, and only in early 1920 did both sides engage in all-out war, the conflicts that took place in 1920 were an inevitable escalation of fighting that began in earnest a year earlier. In the end, the events of 1920 were a logical, though unforeseen, consequence of the 1919 prelude.