Houthi rebels have been attacking ships in the Red Sea for months. Now the U.S. and its allies have given the Yemeni militant group a final warning to stop the attacks. WSJ's Nancy A. Youssef explains what's motivating the Houthis and what the attacks mean for a region already on edge. Further Reading: -U.S., U.K. Warships Shoot Down Houthi Barrage in Red Sea  -Importers Face Surging Shipping Costs, Delays as Red Sea Diversions Pile Up  -U.S., Allies Give Houthis Ultimatum: Stop Ship Attacks or Face Consequences  Learn more about your ad choices. Visit megaphone.fm/adchoices