The struggle during World War II with the Allies was that they were on the same side but weren’t necessarily working toward the same goals. There was the overarching goal of defeating the Germans and liberating Europe from the grasp of Hitler but each side wanted to be the first to reach Berlin. Their secondary goals included obtaining spoils of war, including technologies and Human Resources such as scientists, establishing colonies and puppet states to maintain control over their sphere of influence, as well as to be able to have more control and say over the dismantling of the Third Reich.
To this end, late in the war the Germans seemed to put more resources into holding back the Russians in an attempt to receive more favorable treatment from the Western Allies. True to form in the game, the players struggle against each other to wage the war on their terms. The Western Allies of the US and UK desperately want to stay out of Europe until they are prepared and the Russians want to force commitment to a two Front war so the Germans have to split their forces thereby making their path to Berlin a little easier.
In the picture below, you can see the results of the Russians having to go it alone for too long as their early advance toward Berlin was slowed. Before the Allies conduct Operation Overlord and invade Normandy, the Germans send all of their available troops to defense against the Eastern Horde and routinely in the first several rounds, the Russians are having to roll 3 or less on a 10 sided die in order to be victorious in battle. As the 2nd front is opened, the Germans split their forces and this allows a great chance of success for the Russians as they will have to roll 6 or 7 or less on that same 10 sided die. This allowed the US/UK joint forces to overwhelm the German resistance reaching Berlin first gaining 8VP as compared to the Russians only making it to Prussia for 2VP.