There is no 'if' for 'the US can't/wont provide security'. Just look at the Gulf countries (or Ukraine, for that matter).

The whole 'security' was by intimidation, there is no actual meat on that bone.

Its certainly something to worry about, but i think its far from certain. Nato countries will probably shift to france. The non-nato countries strongly in the us sphere of influence are probably the biggest wories (e.g. japan, south korea) but there aren't actually that many of them.