Here's what we know
According to the United States, Iranian forces shot down an RQ-4A Global Hawk drone that was flying in international airspace over the Strait of Hormuz, the entrance to the Persian Gulf. The U.S. called this an "unprovoked attack" by the Iranian Revolutionary Guard Corps.
Iran says that the drone had flown into Iranian airspace before it was targeted. It said that the drone had been shot down to send a "clear message" to the United States. The IRGC said that the drone "violated Iran's airspace and engaged in information-gathering and spying."
"We do not have any intention for war with any country, but we are fully ready for war," IRGC Gen. Hossein Salami said. The IRGC claimed that after it was shot, the drone crashed in the Jask district of Iran's Kouh-e Mobarak region, about 750 miles from the capital city of Tehran.
U.S. Central Command spokesman Navy Capt. Bill Urban told USA Today that "Iranian reports that the aircraft was over Iran are false." He called this an "unprovoked attack on a U.S. surveillance asset."
U.S. officials told Fox News that Iran had tried to shoot down another drone as well, but had missed.
President Donald Trump tweeted later Thursday morning that Iran had "made a very big mistake!"
Tensions between Iran and the US have been rising
Trump pulled the U.S. out of the nuclear agreement with Iran that had been signed under former President Barack Obama and re-instituted previously lifted sanctions, but relations between the two nations have been strained for a long time now.
In December 2015, Iran test launched a rocket near U.S. warships in the Strait of Hormuz. In January 2016, Iran detained 10 U.S. sailors after the vessel they were in unintentionally entered Iranian waters.
In April, the U.S. designated the IRGC as a terrorist organization. In May, the U.S. deployed a carrier strike group and a bomber task force to the U.S. Central Command region, which includes Iran, in order to "sent a clear an unmistakable message to the Iranian regime that any attack on the United States interests or on those of our allies will be met with unrelenting force." That same month, U.S. military officials accused Iran of using surrogates and proxy forces to attack U.S. forces in the Middle East.
On May 19, Trump tweeted, "If Iran wants to fight, that will be the official end of Iran. Never threaten the United States again!"
U.S. officials have also said that they suspected Iran of being responsible for an attack on a Japanese oil tanker in the Strait of Hormuz just last week.