You can cross earlier than that. You can attack the USA with all 3 Axis. They really can’t stop you from going over there, taking Alaska, annihilating his fleets, and mobbing the coasts. Once you have an LZ, you can fly all the axis air over from all 3 teams. the problem is that you can’t take his territory over either because the USA will turtle up hard. So, you could still SBR him or convoy him, you won’t have what it takes to smash 70 IPCs a turn of basic land units.
The problem with attacking the USA, even with a partial commit, is that while you are taking the USA’s power out of Old World,
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Russia goes crazy against Germany. If Germany’s air and navy are elsewhere, 4 turns of Russian build up changes it from a meager, weak force into an armored titan. Because of the Russian bonus for taking Axis stuff, the Axis can’t focus their defense and Russia rages.
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Japan abandons the continent. When Japan is focused on Hawaii Alaska or the canal, the little guys move in and take over all of Japan’s coastal income.
Keep in mind that the Axis powers can “pass at peace” and move through UK and US blockers, since for the first 3 turns, they are potentially neutral (as long as you abide by the other placement restrictions)