BEIRUT — President Trump’s tough talk on Iran is winning him friends in the Arab world, but it also carries a significant risk of conflict with a U.S. rival that is now more powerful than at any point since the creation of the Islamic republic nearly 40 years ago. With its warning last week that Iran is “on notice,” the Trump administration signaled a sharp departure from the policies of President Barack Obama, whose focus on pursuing a nuclear deal with Iran eclipsed historic U.S. concerns about Iranian expansionism and heralded a rare period of detente between Washington and Tehran.