Under him and with his leadership we can make isolationism as dead an issue as slavery [ 😬 ]
At that time, in that place and context, "isolationism" meant "we don't want to get involved in the war in Europe." That was literally what Luce was talking about. It wasn't about proxy wars in Viet Nam, it was about whether we should let Hitler do his thing in Europe and Japan do their thing in the Pacific and we'd be fine and safe in America, so why bother getting involved?
To put it in modern terms, should we be helping Ukraine defend themselves against Russia?