(The way I play Germany:)
Create 1 huge army that marches into Russia and arrives in Bryansk G5, ready to attack Moscow G6, together with purchases from G5 (aircraft in Berlin/fast in Ukraine). Purchases in G1/2/3/4/5 are always aimed at maximally reinforcing this marching ‘deathball’, like ghr and Demandr described.
Russia has no choice but to surrender Leningrad+Ukraine and Germany should gladly take them with some token forces (1 to 2 inf + air).
What the Allies CAN do about this (very generally speaking because of all the variables): Russia retreats into Moscow while building a lot of inf + ART(!). RAF helps to defend it. UK + USA getting strong at Gibraltar/London + Middle East and, very important, setting up ‘bomber command’. Comes G6 Germany cannot attack Moscow because it is too heavily defended. Luftwaffe is tied into Russia (making life easier for the allies preparing for DDay) because otherwise Russia begins counter-attacking, ending the German presence there. Germany can take Caucasus + Stalingrad but NOT in force because then the Russians can break out where it is most effective, attacking and destroying the German army defending Bryansk if there is still some there. If there is not, because all the Germans are in Caucasus/Stalingrad, what is to stop Russia from retaking Leningrad, Ukraine and all those juicy eastern-european countries? Nada.
Too bad this game has no ‘out-of-supply’ rules as this is exactly what happend in Russia, starving the massive German army in Stalingrad.
In the pacific, the USA + ANZAC have Allied task number 1: prevent an Axis (Pac 6VC) win. This does not mean Japan has to be killed (though you can certainly try), but Hawai + Sydney must be held. If Japan crushes India this is an easy job. If Japan clearly invests in taking Hawai/Sydney (luckily the USA goes after Japan so can react), the task is more difficult, but India will be safe longer and can even become strong strong strong if Japan focuses too much on Hawai. Most Japanese players I have seen, invest heavily into taking India and Middle East and THEN they try to take Hawai/Sydney. This gives the USA breathing space to invest around 20% per turn into the Pacific, spending 80% of its income into Europe. By the time Japan is ready to invest into taking Hawai the USA can switch to near 100% into Pacific (depending on the Japanese investments) without a problem because its forces in Europe are already massive.
Note that I am using the LL system in my games (and posts) to predict/dictate what major powers must do to stay alive. The LL system is usually pretty accurate and if the dice screw you up, well, then it’s the dice, not strategy, that cost you the game.