Which brings us to another isolationist idea: that what we do abroad takes away from what we have, and can spend, at home. When Barack Obama claims, dishonestly, that the cost of foreign wars is guilty of “helping to explode our deficits and constraining our ability to nation-build here at home,” he is sounding this theme. So is Mr. Paul when he demagogues against foreign aid by insisting that “while we are trying in vain to nation build across the globe, our nation is crumbling here at home.”

Republicans should know that deficits are exploding not because of military spending or foreign aid—as a percentage of GDP, George W. Bush spent less on defense in 2008 than Jimmy Carter did in 1980—but because of the growth of entitlement programs. Republicans should know, too, that investing in global order deters more dangerous would-be aggressors and creates a world congenial to American trade, security and values. One cost-effective way of doing that is making an example of a thug who flouts U.S. warnings and civilized conventions.

Taft couldn’t understand this when it came to the dictators of his day. Neither does Mr. Paul when it comes to the dictators of today. The junior senator from Kentucky may not know it yet, but, intellectually speaking, he’s already yesterday’s man. Republicans follow him at their peril.