The assassinations of Iran’s senior leaders by Israel and the United States have triggered unprecedented churn within Tehran’s political and military establishment, eliminating the supreme leader and some of the most powerful men in the Islamic Revolutionary Guard Corps, but have left in place a hard-line government and little hope of a diplomatic breakthrough, according to regional and Western officials. Rather than usher in what President Donald Trump has called “more reasonable” leadership, the surviving Iranian regime is newly emboldened to inflict economic pain, pushing Tehran and Washington further apart in negotiations, according to the officials who spoke on the...