Despite initial US hope for a swift and decisive ouster of Tehran's clerical leadership, Iran kept up retaliatory strikes against Israel and around the Gulf, and vowed to select a new supreme leader to succeed Ayatollah Ali Khamenei, who was killed in the opening salvos on Saturday.
As the war headed into the fifth day, global markets reacted to mounting concerns of a prolonged conflict in a region along a major corridor of oil and gas exports. Gold prices and the US dollar climbed on safe-haven demand.
Airline and tourism industries scrambled to deal with more than 20,000 flight cancellations and governments rushed to bring stranded travellers home from the Middle East.
US Admiral Brad Cooper, commander of the Central Command leading the campaign, said 50,000 troops, 200 jets and two aircraft carriers were engaged in "24/7 strikes into Iran from seabed to space and cyberspace," with more assets on the way.
Iran's air defences had been badly degraded, its navy had no operational vessels on key waterways after 17 were sunk, and more than 2000 Iranian targets had been hit, Cooper said in a video briefing released on Tuesday evening.
"My overall operational assessment is that we are ahead of our game plan," he said. "In simple terms, we're focused on shooting things that can shoot us."
The Israeli military said early on Wednesday it had begun a broad wave of strikes targeting Iranian missile launch sites, aerial defence systems and command centres used to maintain control across the country.
Air raid sirens sounded warnings and buildings shook as incoming Iranian missiles were intercepted inside Israel, witnesses said.
Iran has called the war an unprovoked attack.
Oil prices continued to climb on Wednesday as Tehran attacked ships and energy facilities, closing navigation through the Strait of Hormuz, forcing production stoppages in countries from Qatar to Iraq and halting exports.
The Strait is under the "full control" of Iran's navy, its Revolutionary Guards said on Wednesday, warning vessels not to transit the waterway, according to the semi-official Fars news agency. Its claim of having struck 10 tankers that ignored warnings could not be independently verified.
US President Donald Trump said the US Navy could begin escorting oil tankers through the Strait if necessary. Ship owners and analysts were uncertain that would be enough to stop rising prices or whether the US can spare vessels to do so without exposing them to potential attacks.
Trump risks political fallout if higher energy prices persist, as his Republican Party tries to retain power in the congressional midterm elections in November.
The US strikes against Iran already face headwinds at home and from key Western allies, with rising uncertainty over how long the conflict would last and who would take over control of Iran if the clerical leadership backed by the Revolutionary Guards collapses.
German Chancellor Friedrich Merz told reporters after speaking with Trump in Washington the plan to bring about political change in Iran, which would be positive for the Iranian people, "is not without risk".
French President Emmanuel Macron said he was sending an aircraft carrier to the Mediterranean to help clear maritime traffic imperilled by the crisis but added: "the United States of America and Israel have decided to launch military operations; they were conducted outside international law, which we cannot approve."
In Washington, Trump's top national security advisers spent much of Tuesday making the case to Congress, as some members of his party joined Democrats in saying the president should not send troops to fight abroad without its approval.
The US military on Tuesday identified four of the first American soldiers killed in the war, as the administration warned the conflict would lead to more US casualties. Trump has not ruled out using ground forces.
Iran said deaths from the attacks had reached 787 on Tuesday. That included 165 girls killed on the war's first day when their school was bombed, the highest toll among several civilian sites reported to have been hit.
Trump denied Israel forced his hand into striking Iran and said he had ordered the campaign because he had "a feeling" Tehran would attack after negotiations over its nuclear program stalled.
As Iranians have fled cities, the capital Tehran has become a ghost town.
