The USSR was at its most powerful immediately following the Second World War. The Soviet High Command had learnt from mistakes it made against the Germans (an attack Stalin was completely unprepared for, hence the high Soviet losses) and had mobilised its huge manpower. Further, by 1946, the USSR had consolidated its power over the Eastern bloc, securing and extending a buffer zone to defend the mainland USSR from any Western attack. Given the significant Communist movements in East Asia (Mao in China) and Europe (in France, West Germany, and the Nordic Countries), Stalin would easily have been able to take and hold most of Western Europe, cutting off any foothold the US had to counter-attack.
But seriously Russia wasnt as great as everyone thought they were... Germans killed them 4-1 at the beginning of the war, encircling and destroying entire russian armies. Stalin was quoted as saying without American production, we would have lost the war. We needed the second front with germany to win--but Stalin knew that American equipment made their offensive campaigns possible, and that English aid was valuable in the battle of Moscow. Without Western assistance... I really think that Russia would have lost to germany. If at the end of the war before Russia was fully independent and could fight the coming cold war, I think they would not have been able to fuel their war economy and, over time, the Western democracies would have either 1) won the war, or 2) pushed them out of mainland europe and forced a stalemate.