To provide for European security! That’s the deal in terms of Europe and NATO and also specifically to handle Germany. The idea was that America would provide security to Europe including the nuclear umbrella, and one benefit - among many others - was that Germany would not need to have a powerful military.
Can you perhaps guess why people might be concerned about a heavily armed Germany in the postwar period? Those same concerns are bubbling up in European capitals right now, as Germany rearms due to the loss of the US as a reliable partner.