It sounds like you believe that the Axis can’t withstand the USA entering the war no matter what. Perhaps that has been the case in your games. It also depends on how good a player you have playing the USA. If he/she is really good, and the Axis players are maybe average or less, then of course once the USA gets into the war, with it’s huge income and good build-up of forces during the first three rounds, they are going to clean the Axis’ clocks. A number of people have suggested that USA is too strong economically for a balanced game, myself included. Still, if you have pretty decent Axis players, the USA can be held in check long enough to win the war. One thing I really like is the new NO system for USA so instead of one big 30 IPC NO, they now have 5 or 6 5 IPC NOs that each depend on certain circumstances. This way even America’s great economic might can be limited some.
My Axis gaming friends usually play it to leave USA neutral for as long as possible, ie the first 3 rounds, and then try to deal with them and hope we have UK, ANZAC and Russia beaten down enough that we can at least hold the USA off while trying to capture victory cities to win the game. Sometimes it works, sometimes it doesn’t.
Recently, we played a game where Japan decided to go all out and attacked everything but Russia on J1, thus bringing USA into the war right away. Believe it or not, the Axis ended up winning that game and it sort of seemed like USA was easier to handle dealing with them from round 1 than letting them sit for 3 rounds. Sure, they had a much larger income right away, but they also had less military units to start with. Then again, maybe our USA player was just too surprised by the early attack and couldn’t figure out his strategy. He might have been too used to the 3 round build up.