Yes, defending Russia is THE most important issue for the Allies in this game. Here is a guide to what you need to do to protect Moscow.
Russia: You should buy max bodies every turn. This means all infantry all the time (Assuming full Barbarossa). 37 PU’s should become 11 inf and an art or mech. The most common move Germany will do is ball up a large stack, attack on G3 and march straight towards Moscow. You need to be in place in Bryanks and then Moscow to confront this stack. YOU CAN NOT WASTE INFANTRY TRYING TO GUARD NOVGOROD, they will not make it home for the Moscow fight in time if you leave them behind even for a turn. The 2nd major thign you need to do is secure income that will insulate your production when Germany tries to seige you after you block the G6-8 hit. This is done by sending you starting fast units (2 mech and 2 tanks) to hit Iraq, preferably on R3. If it is full Barbarossa you need to stack the **** out of Moscow and wait for your Allies to equalize the game. If Germany attempts to move his stack to another area, then you can step out with your stack and make some noise.
UK: Your opening bid will be important for shutting Italy down early and this Allows you to shift from Africa/Med to helping Russia by turn 3-5. I Recommend bidding 13 and putting an art in Alexandria, a tank in Egypt and an Infantry in Anglo-Egypt Sudan and killing Tobruk on UK1. Don’t commit your tranny to the Tobruk fight, as you will need it to grabe Persia and turn 1. It is very important that UK land in Persia on turn 1 for two reasons. First, it puts you infantry in range in order to open NW Persia so the Russian fast (the 2 tanks and Mechs) can move threw and capture Iraq on R3. Secondly, you will want to place a British factory in Persia by turn 2-3. This factory will be used to fly planes to Moscow and also produce infantry so Germany can’t just waltz into Persia and Iraq and collect their oil money. I find it important to wrap up thing in the Med and Africa by turn 3-4 at the latest, because by turn 5-6 you need to be sending help to Russia ASAP. After Moscow is safe it still doesn’t completely fix the problem because Germany will be making 70+ and continuing to produce ground untis that you cannot match easily. The final stage of stopping Germany is taking you victorious med fleet out of the med and linking with the USA fleet which should be sizeable enough to land in Europe by turn 4-6, depending on how much USA is committing to the PAC.
USA: In general, i prefer a 80%-20% split for USA’s resource between the pac and Europe respectively. But this can very depending out which theater you wish to control the most. USA should build a fleet large enough that the combined USA/UK fleet can not be attacked by the German air force. Once this is done, link with the UK fleet in SZ 91 and build trannies and art/inf to land in Europe. From SZ 91 it is just a matter of picking your flavor. You essentially have 3 options. 1. you can take both fleets and land in Norway. Securing Norway is huge because it cuts 8 PU from Germany’s income every turn. 2. You can move the fleets to SZ 110 and begin trading Holland and Normandy with the Axis. Do this lightly at first just to bleed German units away from Russia, but begin to to it harder if you feel Germany is committing enough to respect your threat. 3. This is the least effective option in general IMO, but when you are in SZ 91 you should have a credible threat on Rome. If the Axis don’t proctect Rome with enough units, dive on it with a 1-2 punch from UK/USA, thus plundering an Axis capital. In general though this can be hard to do, and Italy will be shut down pretty hard just by putting 5 subs into SZ 97 and cutting the majority of their income.
Conclusion: Even doing all this sometimes isn’t enough because it can be tough for USA to spend enough resources in Europe to make this happen while containing Japan. And the strategy is contingent on gettign an early knockout on Italy in the med, which usually happens but not every game. But, if you’re able to execute it correctly Moscow should be safe for a considerable time. My buddies I play against are expert Barbarossa players and i rarely lose Moscow before turn 10+ if I do at all. So i hope this helps!