As long as they go somewhere else to do their fighting.
A government that allowed its own citizens to enlist in the armed forces of a foreign occupying power - even if it calls itself a foreign assisting power - for domestic service would lose even the most tattered facade of sovereignty or legitimacy. It would no longer be able to even pretend to be a government.
The idea that the US could recruit, say, Afghan units into the US military to serve in Afghanistan is perhaps an interesting speculation, but hardly relevant to any real-world situation.
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