What are the disadvantages to protectionism?

There are any number of disadvantages to an isolationist foreign policy. It can create a false sense of security in that foreign wars seem to be avoided by being isolationist. With that said, that may be a false sense of security as WW 2 demonstrated.

While a nation may be able to avoid unfavorable alliances, it also precludes favorable ones. Other nations normally of a friendly nature have no obligation to help nations that insist on isolationism. It also may not be a deterrent to powerful nations that can exploit isolationist nations as they seem to have no friends.