Iran has limited options to respond directly. The danger, however, is that Israel has opened a Pandora’s box: the worst Iranian response might also be the most likely, a decision to withdraw from its arms control commitments and build nuclear weapons in earnest.

Mr. Netanyahu said Israel was facing “difficult days, but great days” ahead. He also repeatedly invoked the Holocaust — the annihilation of European Jewry — as a reason not to treat a nuclear Iran lightly. “Together, with God’s help, we will ensure Israel’s eternity,” he said.

The Israeli military has begun airstrikes against Iran, two U.S. officials said Thursday evening, a dramatic escalation that increased the chances of an all-out war between the countries and expanding the long-running regional conflict. There is no U.S. involvement or assistance, the officials said.