ISTANBUL—The conflict between the U.S. and Iran is escalating just as the two warring parties are running up against the limits of what they can achieve through military force.The conflict widened this week after Iranian-allied militants in Yemen began striking ships transiting a key Red Sea chokepoint, helping drive oil prices above $100 a barrel for the first time in two months on Thursday.The U.S., meanwhile, has escalated its bombing campaign in Iran in recent days, striking throughout the country during the most intense period of fighting since President Trump and Israel began bombing Iran in late February.Trump and other...