Faced with U.S. demands to provide the military muscle to enforce a future Ukraine peace deal, Europe finds itself in a bind.

Experts say that sending European peacekeepers to Ukraine might stretch and weaken NATO's own defenses, and that the mission would still need U.S. support to succeed.

While U.S. boots on the ground may not be necessary, deterrence in the form of U.S. medium-range missiles and ultimately nuclear weapons will remain crucial.