President Trump ordered the killing of the powerful commander of Iran’s Revolutionary Guards Corps, Maj. Gen. Qassim Suleimani, in a drone strike on the Baghdad International Airport early Friday, American officials said. General Suleimani’s death was confirmed by official Iranian media. “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.” “This strike was aimed at deterring future Iranian attack plans,” the statement added. “The United States will continue to take all necessary action to protect our people and our interests wherever they are around the world.” The killing of General Suleimani was a major blow for Iran and a major escalation of President Trump’s “maximum pressure” campaign against Iran, which began with economic sanctions but has steadily moved into the military arena. The strikes followed a warning on Thursday afternoon from Defense Secretary Mark T. Esper, who said the United States military would pre-emptively strike Iranian-backed forces in Iraq and Syria if there were signs the paramilitary groups were planning more attacks against American bases and personnel in the region. “If we get word of attacks, we will take pre-emptive action as well to protect American forces, protect American lives,” Mr. Esper said. “The game has changed.” In Iran, state television interrupted its programing to announce General Suleimani’s death. The news anchor recited the Islamic prayer for the dead — “From God we came and to God we return” — beside a picture of General Suleimani. American officials consider General Suleimani, the commander of the Revolutionary Guards’ elite Quds Force, responsible for the deaths of hundreds of American soldiers during the Iraq War and hostile Iranian activities throughout the Middle East. “This is devastating for the Iranian Revolutionary Guards Corps, the regime and Khamenei’s regional ambitions,” said Mark Dubowitz, chief executive of the hawkish Foundation for Defense of Democracies, a think tank that supports a hard line against Iran, referring to the country’s supreme leader, Ayatollah Ali Khamenei. “For 23 years, he has been the equivalent of the J.S.O.C. commander, the C.I.A. director and Iran’s real foreign minister,” Mr. Dubowitz said, using an acronym for the United States Joint Special Operations Command. “He is irreplaceable and indispensable” to Iran’s military establishment. The American drone strike hit two cars carrying Mr. Suleimani and several officials with Iranian-backed militias as they were leaving the airport. American officials said that multiple missiles hit the convoy in a strike carried out by the Joint Special Operations Command. The strike killed five people, including the pro-Iranian chief of an umbrella group for Iraqi militias, Iraqi television reported and militia officials confirmed. The militia chief, Abu Mahdi al-Muhandis, was a strongly pro-Iranian figure.The public relations chief for the umbrella group, the Popular Mobilization Forces in Iraq, Mohammed Ridha Jabri, was killed as well. Two other people were killed in the strike, according to a general at the Baghdad joint command, who spoke on condition of anonymity because he was not authorized to speak to the news media.