President Trump’s decision to strike and kill the second most powerful official in Iran turns a slow-simmering conflict with Tehran into a boiling one, and is the riskiest move made by the United States in the Middle East since the invasion of Iraq in 2003.

The calculus was straightforward: Washington had to re-establish deterrence, and show the Iranian leadership that missiles fired at ships in the Persian Gulf and at oil facilities in Saudi Arabia, along with attacks inside Iraq that cost the life of an American contractor, would not go without a response.

But while senior American officials have no doubt the Iranians will respond, they do not know how quickly, or how furiously.

For a president who repeated his determination to withdraw from the caldron of the Middle East, the strike that killed Maj. Gen. Qassim Suleimani, who for two decades has led Iran’s most fearsome and ruthless military unit, the Quds Force, means there will be no escape from the region for the rest of his presidency, whether that is one year or five. Mr. Trump has committed the United States to a conflict whose dimensions are unknowable, as Iran’s supreme leader, Ayatollah Ali Khamenei, seeks vengeance.