The headline is vague. What I mean by "traditionally" means disband nearly all of the standing armed forces, no permanent peacetime alliances, and not much involvement in world affairs outside of North America and the Pacific (the occupation of Japan still happens because that is actually in line with nineteenth century US policies). Pretty much nothing that Washington wouldn't have approved of.
Simplest POD is something like a Byrnes administration, followed by a Robert Taft administration after 1948. Or maybe Roosevelt dies earlier and Taft defeats Wallace in 1944. What happens?