<P> Successes at Operation Bagration and in the year that followed were, in large part, due to an operational improve of battlehardened Red Army, which has learned painful lessons from previous years battling the powerful Wehrmacht: better planning of offensives, efficient use of artillery, better handling of time and space during attacks in contradiction to Stalin's order "not a step back". To a lesser degree, the success of Bagration was due to a weakened Wehrmacht that lacked the fuel and armament they needed to operate effectively, growing Soviet advantages in manpower and materials, and the attacks of Allies on the Western Front . In his 1944 May Day speech, Stalin praised the Western allies for diverting German resources in the Italian Campaign, Tass published detailed lists of the large numbers of supplies coming from Western allies, and Stalin made a speech in November 1944 stating that Allied efforts in the West had already quickly drawn 75 German divisions to defend that region, without which, the Red Army could not yet have driven the Wehrmacht from Soviet territories . The weakened Wehrmacht also helped Soviet offensives because no effective German counter-offensive could be launched, </P> <P> Beginning in the summer of 1944, a reinforced German Army Centre Group did prevent the Soviets from advancing in around Warsaw for nearly half a year . Some historians claim that the Soviets' failure to advance was a purposeful Soviet stall to allow the Wehrmacht to slaughter members of a Warsaw Uprising by the Polish home army in August 1944 that occurred as the Red Army approached, though others dispute the claim and cite sizable unsuccessful Red Army efforts to attempt to defeat the Wehrmacht in that region . Earlier in 1944, Stalin had insisted that the Soviets would annex the portions of Poland it divided with Germany in the Molotov - Ribbentrop Pact, while the Polish government in exile, which the British insisted must be involved in postwar Poland, demanded that the Polish border be restored to prewar locations . The rift further highlighted Stalin's blatant hostility toward the anti-communist Polish government in exile and their Polish home army, which Stalin felt threatened his plans to create a post-war Poland friendly to the Soviet Union . Further exacerbating the rift was Stalin's refusal to resupply the Polish home army, and his refusal to allow American supply planes to use the necessary Soviet air bases to ferry supplies to the Polish home army, which Stalin referred to in a letter to Roosevelt and Churchill as "power - seeking criminals". Worried about the possible repercussions of those actions, Stalin later began a Soviet supply airdrop to Polish rebels, though most of the supplies ended up in the hands of the Germans . The uprising ended in disaster with 20,000 Polish rebels and up to 200,000 civilians killed by German forces, with Soviet forces entering the city in January 1945 . </P> <P> Other important advances occurred in late 1944, such as the invasion of Romania in August and Bulgaria . The Soviet Union declared war on Bulgaria in September 1944 and invaded the country, installing a communist government . Following the invasion of these Balkan countries, Stalin and Churchill met in the autumn of 1944, where they agreed upon various percentages for "spheres of influence" in several Balkan states, though the diplomats for neither leader knew what the term actually meant . The Red Army also expelled German forces from Lithuania and Estonia in late 1944 at the cost of 260,000 Soviet casualties . </P> <P> In late 1944, Soviet forces battled fiercely to capture Hungary in the Budapest Offensive, but could not take it, which became a topic so sensitive to Stalin that he refused to allow his commanders to speak of it . The Germans held out in the subsequent Battle of Budapest until February 1945, when the remaining Hungarians signed an armistice with the Soviet Union . Victory at Budapest permitted the Red Army to launch the Vienna Offensive in April 1945 . To the northeast, the taking of Belarus and western Ukraine permitted the Soviets to launch the massive Vistula--Oder Offensive, where German intelligence had incorrectly guessed the Soviets would have a 3 - to - 1 numerical superiority advantage that was actually 5 - to - 1 (over 2 million Red Army personnel attacking 450,000 German defenders), the successful culmination of which resulted in the Red Army advancing from the Vistula River in Poland to the German Oder River in Eastern Germany . </P>

What was the role of the soviet union in ww2