During the Polish–Soviet War of 1919-1921, Soviet Russia and Soviet Ukraine were in combat with the re-established Second Polish Republic and the newly established Ukrainian People's Republic. Both sides aimed to secure territory in the often disputed areas of the Kresy (present-day western Ukraine and western Belarus), in the context of the fluidity of borders in Central and Eastern Europe in the aftermath of World War I and the breakdown of the Austrian, German, and Russian empires. The first clashes between the two sides occurred in February 1919, but full-scale war did not break out until the following year. Especially at first, neither Soviet Russia, embroiled in the Russian Civil War, nor Poland, still in the early stages of state re-building, were in a position to formulate and pursue clear and consistent war aims. In the winter of 1918-19 the recently established Russian Soviet Republic had already undertaken a "Westward Offensive", leading to the establishment of Soviet Republics in Latvia, Lithuania and Belarus. The overarching aim of Lenin and Trotsky was to spread the Bolshevik revolution into Germany and other parts of Europe, while realising their limited capacity to engage in large-scale conflict in Europe. The Soviets built up forces positioned for an attack on Poland, although officially denying that an invasion was planned. For their part, the Poles aimed to secure their independence, contain any resurgence of the Russian or German Empires, and reclaim areas in the east that had ethnic Polish populations as per the historical Polish–Lithuanian Commonwealth borders that existed prior to the Partitions of Poland. An ambitious plan formulated by Polish head of state Józef Piłsudski aimed to create a wide "Intermarium" (Międzymorze) federation of independent states to the west of Russia, of which Poland would be the leading member. Limiting Russian control of Ukraine was essential to this plan, and Poland was already at war with Ukrainian forces over Eastern Galicia. As 1919 progressed, Soviet forces conquered Kiev. In early 1920, Poland formed an alliance with the Ukrainian People's Republic, which had lost much of its territory to the Bolsheviks. Both Polish and Soviet forces in the theatre were rapidly increased, and full-scale war began with Poland's Kiev Offensive into Soviet-controlled Ukraine. The prehistory The territory, where this conflict broke out, was a part of the medieval Kievan Rus', and after the disintegration of this united Ruthenian state (in the middle of the 12th century)