President Trump has set aside, at least for now, plans to sharply escalate the American military assault against Iran, with particular concern that intensifying the war could dangerously drain the Pentagon’s already diminished stockpile of Patriot antimissile interceptors and other air defense munitions in the Middle East. The threat to interceptor stockpiles is one of many considerations that have made a return to major combat operations a hugely risky endeavor, administration officials say. Mr. Trump and his top aides are also uneasy about the prospect of a widening war in the Middle East, the alienation of key Gulf allies who...