Israel’s unrelenting assault on the Gaza Strip is beginning to tip the Middle East into a wider regional conflict. In the past week, the Houthis in Yemen emerged as an unlikely power player, successfully disrupting global shipping in the name of Palestinians in Gaza and goading the U.S. into launching a series of airstrikes in a failed bid at deterrence. Over the past three months, the Houthis have attacked merchant ships passing through the Red Sea, an unexpected military intervention aimed at forcing Israel to end its U.S.-backed offensive in Gaza and allow aid into the besieged territory. The Houthis’...