Ben Wallace said the prospect of the US stepping back from its international leadership role under Donald Trump "keeps me awake at night".
It may force the UK to rethink its assumptions about defence, he added.
His comments come as the UK prepares to carry out the "deepest review" of Britain's security, defence and foreign policy since the end of the Cold War.
"I worry if the United States withdraws from its leadership around the world," Mr Wallace told the Sunday Times. "That would be bad for the world and bad for us. We plan for the worst and hope for the best."