President Trump on Thursday ordered the U.S. military to conduct a limited strike against Iran to retaliate for the downing of a Navy surveillance drone in the Persian Gulf region, but he then called off the operation in the evening for unknown reasons. The president's change of course avoided, for now, what would have been the most significant escalation of hostilities between Washington and Tehran in decades.

There was every indication the U.S. was about to launch a limited strike against Iranian surface-to-air missile sites on Thursday evening, but the military stood down at the 11th hour for still-unexplained reasons. More than one source confirmed this to CBS News, including two sources within the Trump administration who were familiar with the president's decision. The sources said the plan was already underway when it was called off.

The New York Times first reported that Mr. Trump approved the strike and then pulled back before it was carried out.

The Pentagon announced earlier on Thursday that a U.S. drone flying above international waters in the Strait of Hormuz had been shot down by an Iranian surface-to-air missile system on Wednesday night. The Trump administration denounced the downing as an "unprovoked attack," while the government in Tehran accused the U.S. of being the aggressor and said the drone was flying in Iranian airspace.

Iran stuck to its line on Friday morning, claiming "indisputable" evidence that the U.S. spy drone had violated its airspace. Deputy foreign minister Abbas Araghchi told the Swiss ambassador, whose country represents U.S. interests in Iran, that Tehran had collected wreckage from the Global Hawk "from Iran's territorial waters." He said Iranian would present its evidence to the United Nations.  

Although the president suggested the drone had been shot down by "mistake," he nevertheless warned that the U.S. would respond to any belligerent moves by Tehran. 

"This country will not stand for it," Mr. Trump told reporters at the White House before convening an emergency meeting with Republican and Democratic leaders in Congress. 

House Speaker Nancy Pelosi, who met with the president, urged him to obtain congressional authorization to use any kind of military force. She said she couldn't predict whether the president would consult Congress before taking military action, but did note that lawmakers had the opportunity to share their thoughts with the administration Thursday. She also said the president told them they would have more meetings about the issue.

The discussion on Thursday within the Trump administration on whether to take retaliatory military action represents the latest flash-point in nearly 40 years of geopolitical enmity between both countries that began after the U.S.-allied monarchy was overthrown during the Iranian Revolution in 1979. 

The strained relationship entered a détente of sorts under the Obama administration, which allied with European countries to broker a landmark deal with Iran to reduce its medium-enriched uranium stockpile in exchange for the lifting of economic sanctions, but it has worsened since Mr. Trump took office. 

Although America's European allies are still abiding by it, Mr. Trump withdrew the U.S. from the agreement, dubbed the "Iran nuclear deal," in May 2018, deriding it as a one-sided deal. He has also imposed crippling sanctions on Iran. 

Mounting tension in recent weeks has raised fears that a full-blown conflict could ensue between the countries as hardliners in both Tehran and Washington escalate their rhetoric.

Earlier in the month, the Trump administration accused the Iranian government of attacking two oil tankers in the Gulf of Oman, a vital waterway near the Arabian Peninsula for international oil shipments. Officials said U.S. intelligence suggested the Islamic Revolutionary Guard Corps — an elite military unit which answers directly to Supreme Leader Ayatollah Ali Khamenei — was behind the attacks.