Iran’s acknowledgement that it shot down a Ukrainian airliner, killing 176 people, raises new challenges for the Islamic Republic both externally amid tensions with the U.S. and internally as it deals with growing discontent from its people. The country did itself no favors by having its air-crash investigators, government officials and diplomats deny for days that a missile downed the flight, though a commander said Saturday that he had raised that possibility to his superiors as early as Wednesday, the day of the crash. While its paramilitary Revolutionary Guard took responsibility, the same commander claimed it warned Tehran to close off its airspace amid fears of U.S. retaliation over Iran launching ballistic missiles at Iraqi bases housing U.S. forces.