WEST PALM BEACH, Fla. — Iran’s top security and intelligence commander was killed early Friday in a drone strike at Baghdad International Airport that was authorized by President Trump, American officials said.

The commander, Maj. Gen. Qassim Suleimani, who led the powerful Quds Force of the Islamic Revolutionary Guards Corps, was killed along with several officials from Iraqi militias backed by Tehran when an American MQ-9 Reaper drone fired missiles into a convoy that was leaving the airport.

General Suleimani was the architect of nearly every significant operation by Iranian intelligence and military forces over the past two decades, and his death was a staggering blow for Iran at a time of sweeping geopolitical conflict.

The strike was also a serious escalation of Mr. Trump’s growing confrontation with Tehran, one that began with the death of an American contractor in Iraq in late December.

In Iran, the leadership convened an emergency security meeting. And the country’s supreme leader, Ayatollah Ali Khamenei, issued a statement calling for three days of public mourning and then retaliation.

“His departure to God does not end his path or his mission,” the statement said, “but a forceful revenge awaits the criminals who have his blood and the blood of the other martyrs last night on their hands.”

United States officials were braced for potential Iranian retaliatory attacks, possibly including cyberattacks and terrorism, on American interests and allies.

Israel, too, was preparing for Iranian strikes. Some of the country’s most popular tourist sites, including the ski resort at Hermon, were closed, and the armed forces went on alert, officials said.

From the start of the Syrian civil war, General Suleimani was one of the chief leaders of an effort to protect President Bashar al-Assad of Syria — an important Iranian ally — that brought together disparate militias, national security forces and regional powers, including Russia in recent years.

But that was far from the only front he operated on. American officials accuse General Suleimani of causing the deaths of hundreds of soldiers during the Iraq war, when he provided Iraqi insurgents with advanced bomb-making equipment and training. They also say he has masterminded destabilizing Iranian activities that continue throughout the Middle East and are aimed at the United States, Israel and Saudi Arabia.

“General Suleimani was actively developing plans to attack American diplomats and service members in Iraq and throughout the region,” the Pentagon said in a statement. “General Suleimani and his Quds Force were responsible for the deaths of hundreds of American and coalition service members and the wounding of thousands more.”

It did not elaborate on the specific intelligence that led them to carry out General Suleimani’s killing. The highly classified mission was set in motion after the American contractor’s death on Dec. 27 during a rocket attack by an Iranian-backed militia, a senior American official said.

In killing General Suleimani, Mr. Trump took an action that Presidents George W. Bush and Barack Obama had rejected, fearing it would lead to war between the United States and Iran.