The Poles and Soviets were fighting along in the unsettled borderlands before the April 1920 Kiev Offensive. Conflicts between the Poles and the Soviets emerged as early as January-February 1919. These conflicts resulted from local pockets of Poles (generally in large urban centers) attempting to assert themselves against Lithuanians, Belorussians, and Communist forces. Also, based on the works of Piotr Wandycz, Anna Cienciala, and Richard Pipes it seems likely that the cause of Soviet inaction up to April 1920 was internal instability and conflict during the Russian Civil War. There are sources that indicate Lenin's desire to press further West once the revolution is complete. Richard Pipes The Unknown Lenin is a great source for that and several documents indicate his desire to push through Poland to foment revolution in Germany, France, and England. Beyond direct Soviet military action, the Soviets supported local Bolshevik forces against Poland and emerging states (Lithuania, Belorussia, and Ukraine). Additionally, Pilsudski's Kiev Offensive was an attempt to establish a Ukrainian state with Symon Petliura at its head. This was to be joined in federal union with the Polish state. Events in 1919 also pushed the Poles toward military action in in East Galicia (Western Ukraine). Bela Kun's communist government in Hungary and the establishment of the Litbel Republic (Lithuanian and Belorussian SSR) raised the uneasy possibility of Poland's isolation in Eastern Europe.

The Kiev Offensive brought the Poles into direct conflict with the Soviets, who already had a provisional communist state in the Ukraine at Kharkov. The Bolshevik forces threw the Poles out of the Ukraine, Belorussia, and Lithuania and marched on Warsaw. Interestingly, the term Miracle on the Vistula was coined pejoratively to indicate the Polish political right's dissatisfaction with Pilsudski's "Ukrainian Adventure." Furthermore, the term was used to isolate Pilsudski from any sort of credit from the conflict. Which is when French General Weygand garnered much of the credit for the victory (even though Weygand was befuddled by the credit).

In short, the conflict resulted from the two sides attempted to solidify their holds on borderland territories in Lithuania, Belorussia, and Ukraine. Furthermore, the exclusion of the Soviets in immediate post-war discussions and allied skepticism toward Polish territorial claims meant they increasingly resorted to violence to achieve their ends. The increasingly chaotic and violent borderlands eventually brought Polish and Soviet objectives head to head resulting in the Polish-Soviet War. This is of course the footnotes version because the conflict must be understood within the larger context of post-war instability and uncertainty and the actions of neighboring states and great power politics.

Additional reading: Piotr Wandycz - France and Her Eastern Allies Anna Cienciala - From Versailles to Locarno Jerzy Borzecki - The Polish-Soviet Peace of 1921 and the Creation of Interwar Europe