Of course we should have fought Hitler. That is asinine if Buchanan really does think that.
Buchanan does have a point about how we should have stayed out of WWI, and if the Treaty of Versailles were more reasonable (like the peace after WWII), then we wouldn't have had a Hitler in charge of Germany.
But, while hindsight shows we should have tried harder to stay out of WWI, the Zimmerman Telegram kind of sealed the deal, I think. Granted, our aid to the UK led to Germany flirting with Mexico, which makes you realize that aid can cause a war, so it is serious business (and should be handled more behind the scenes than we are currently doing it).