IRBIL, Iraq — The United States intervened militarily in Iraq early this morning for the first time since Islamist militants seized control of nearly half the country, dropping humanitarian supplies to tens of thousands of people who fled their homes when the Islamic State captured the city of Sinjar in northern Iraq.In a nationally televised address, President Barack Obama said he had also authorized targeted airstrikes “to protect American personnel,” citing an Islamic State advance on the city of Irbil, the capital of Iraq’s autonomous Kurdistan Regional Government. But Obama indicated that such strikes had not yet been made and stressed that he did not intend a massive military campaign.At the request of the Iraqi government, Obama said, he acted also to help save Iraqi citizens, many of them religious minorities, who are trapped on a mountaintop in northern Iraq.“When we have the unique capabilities to help avert a massacre, then I think the United States of America cannot turn a blind eye,” Obama said.But he also declared that he had run for the presidency in part to end America’s involvement in the Iraq war, and he repeated his promise that the United States will not send ground troops back to that country.“I will not allow the United States to be dragged into fighting another war in Iraq,” he said.Obama said those stranded on the mountain were subject to “barbaric” treatment by the Islamic State and were threatened with a mass execution “which would constitute a genocide.”The religious minorities, he said, were faced with this choice: “Descend the mountain and be slaughtered, or stay and slowly die of thirst and hunger.”A statement from the Pentagon distributed shortly before Obama spoke said the initial mission undertaken Thursday was limited to bringing food and water to thousands of members of the Yazidi religious minority.A statement from the Pentagon distributed shortly after the president spoke said one C-17 aircraft and two C-130s had dropped a total of 72 bundles of supplies to the thousands of members of the Yazidi religious minority who had fled to mountains near the city of Sinjar after the Islamist extremists seized the city. The supplies included 5,300 gallons of fresh drinking water, the statement said, an especially needed commodity in Iraq’s 115-degree summer heat.Two F/A-18 fighter jets accompanied the cargo planes, but no ground troops were involved, the Pentagon said. All of the aircraft had left the area of the drop by the time the president spoke, the statement said. There were no reports of hostile fire.The mission was announced after a tumultuous day in which the Islamic State captured a string of towns that brought its forces to within minutes of Irbil. Kurdish peshmerga militia rushed to set up a defensive line near the town of Kalak, about 25 miles northwest of Irbil, as Kurdish officials pleaded with the United States for direct military support and supplies.Early this morning, a resident of Kalak told McClatchy that she had heard a jet aircraft overhead and had heard explosions from behind Islamic State lines. But the aircraft was Iraqi, not American, according to Kurdish and American officials.Dying of thirstThe United States dropped supplies to the Yazidis at the request of the Iraqi government, Obama said. The Yazidis, who have been targeted for years by Muslim extremists who consider their religious beliefs heretical, were forced from their homes when the Islamic State captured the city of Sinjar, fleeing to the nearby mountains with little more than the clothes on their back.In recent days, human-rights advocates have pleaded for international help to reach the Yazidis, who lacked drinking water and food. Dozens of people were reported to have died of thirst in the Iraqi heat.Obama said the U.S. military had also been authorized to strike at Islamic State forces if they move against the Yazidi refugees.Likely to be used in the strikes are the USS George H.W. Bush aircraft carrier and a strike group of other ships supporting the massive nuclear-powered carrier named for the 41st president. Bush took the nation into the first Iraq war in 1991, and then his son, George W. Bush, followed with the second conflict in 2003.The new U.S. intervention was almost certain to ease tension in Irbil, which was on edge Thursday after the Islamic State announced in an Internet posting that it intended to capture the city.Irbil no longer safeUntil this week, the Kurdish region had been considered so secure that the United States had chosen it as one of two Iraqi locations safe enough to transfer staffers from the U.S. Embassy in Baghdad. But a sense of dread fell over the Kurdish capital Thursday as the magnitude of the threat became clear.Western oil companies based in Irbil shut down operations and restricted their employees’ movements out of concerns for safety, and makeshift shelters popped up in public parks and churches in the Ain Kawa neighborhood to accommodate hundreds of people who had fled the newly occupied towns. There was a noticeable increase in the presence of the Kurdish peshmerga militia in the city, and there were reports that hundreds of residents flooded the airport in hopes of buying tickets to elsewhere.A refugee camp at Kalak that only two days ago was filled with tens of thousands of refugees who had fled Mosul when it fell to the Islamic State was empty Thursday as the area became the new front line of a conflict that went from occasional clashes to a full-scale war between the Kurds and the Islamic State in less than a week.The peshmerga appeared to be preparing to make a last stand at Kalak. Several hundred regulars in uniforms with well-maintained light weapons and heavy machine guns, backed by a few armored vehicles and a single Soviet-era T-55 tank, were digging in with earth movers along a string of desolate desert hills to prepare for what a top security official called a “very serious test.”“The Americans keep saying they will help us,” said Rosg Nuri Shawess, a top Kurdish military commander who was overseeing the defensive preparations. “Well, if they plan to help they had better do it now.”Shawess said his men were confident and well-trained, an assertion reinforced by the professional demeanor of his uniformed men. But the peshmerga , with just a handful of 12.7 mm Soviet-era heavy machine guns and rocket-propelled grenades, will face Islamic State fighters armed with advanced U.S. weapons.“We need better weapons and help,” Shawess said. “They tried to attack this morning but were just testing us.” Staff writer John Gravois contributed to this report, which includes material from The New York Times.