@Running-Away Per page 22 of the rulebook, units of another power may be transported only if that power is friendly. Per page 12, the US is not considered to be either friend or enemy to any other power while it remains neutral. Therefore, the US cannot transport units belonging to any other power while it is neutral.
African Strats
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I found that moving all the German units into Belgian Congo is a very effective strategy. It forces the Allies to group up and attack you which either results in their complete annihilation or leaves them so weak that the Ottomans will just storm the whole continent with minimal effort