President Biden has been walking a politico-military tightrope since Hamas’ Oct. 7 attack on Israel and Israel’s subsequent bombing of Gaza. But he’s about to traipse an even narrower rope. He needs to respond to an Iranian-backed militia’s drone strike that killed three U.S. servicemen in a decisive‚ powerful way—without widening the war any further. Some—including Sen. Tom Cotton and a few commentators—are urging him to widen the war. Iran is supplying the entire network of militias fueling...