And the US gets to sell their weapons to allies. This acts as a subsidy on the development of new weapons systems, and prevents possible competitors from developing theirs. It is easier to win a war when your opponent cant make any weapons, or cant scale their weapon making fast enough.
It also provides an ever present treat to Europe: All those US bases scattered over the European continent are a reminder that Europe is free to choose their own political path, as long as they don't deviate much from what Washington approves.
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