After a fiery night of Israeli attacks across Iran, followed by a fusillade of Iranian missiles launched at Israeli cities in retaliation, the Middle East awoke Saturday to a radically reshaped landscape, with the combatants digging in.

Prime Minister Benjamin Netanyahu vowed that Israel’s assault would last “as many days as it takes” to eliminate any nuclear threat Iran could pose against Israel. President Trump piled on, casting the stakes in near-apocalyptic terms for Iran.

“Iran must make a deal, before there is nothing left, and save what was once known as the Iranian Empire,” he posted on social media, hours after Israeli jets struck dozens of targets, killing much of Iran’s military high command.

Both men appeared to be gambling: in Mr. Netanyahu’s case, that Israel’s barrage of attacks will fatally damage Iran’s nuclear program and decapitate its military leadership; in Mr. Trump’s case, that the assault will weaken Iran and force it into a diplomatic accommodation with the United States — without spiraling into unintended, potentially catastrophic consequences.

For other world leaders, including Prime Minister Keir Starmer of Britain and President Emmanuel Macron of France, those consequences loomed large. They urged restraint, warning of ripple effects in a region that has already been at war on multiple fronts, from Gaza to the Israeli offensive against Hezbollah in Lebanon and the attacks of the Houthi rebels on shipping in the Persian Gulf.

Israel’s audacious attack will almost certainly torpedo Mr. Trump’s attempts to broker a deal curbing Iran’s nuclear ambitions. His implication that the Israeli attack could be a lever to soften up the Iranian leadership for diplomacy seemed far-fetched in the wake of images of burning apartment towers in Tehran.