The United States and Iran have been lobbing threats, fighting proxy wars, and imposing sanctions for decades. USA Today looks at over 60 years of this back-and-forth.
Just the FAQs, USA TODAY

U.S.-Iranian tensions heightened Thursday when a U.S. surveillance drone was shot down by Iran’s Revolutionary Guard in a strike it said sent a “clear message” that the Persian Gulf nation was ready to defend itself from what it views as Western aggression.

U.S. Defense officials and Iranian authorities provided alternative details on the incident, with Iran saying the drone was brought down over the southern coast of its Hormozgan Province. The Pentagon said the incident happened in international waters over the Strait of Hormuz.

“U.S. Central Command can confirm that a U.S. Navy Broad Area Maritime Surveillance ISR aircraft was shot down by an Iranian surface-to-air missile system while operating in international airspace,” said Navy Capt. Bill Urban, spokesman for U.S. Central Command. “Iranian reports that the aircraft was over Iran are false.”

Urban called the strike an “unprovoked attack on a U.S. surveillance asset.”

War with Iran?: Escalating Iran crisis looks a lot like the path US took to Iraq war

Revolutionary Guard Commander Major General Hossein Salami said Iran does not want war, but that the incident should serve as a warning to the U.S. to stay away.

“The downing of the U.S. drone had an explicit, decisive and clear message that defenders of the Islamic Iran’s borders will show decisive and knockout reactions to aggression against this territory,” Salami said at a news conference in Kurdistan Province. “Borders are our redline, and any enemy violating these borders will not go back.” 

The RQ-4 Global Hawk, built by defense giant Northrop Grumman, is a high-altitude drone capable of flying for more than 30 hours at a time. It is not the drone you see your neighbor flying over your street – this one is more than 47 feet long and has a wingspan of more than 131 feet. The military has flown them since 2001.

The Iranian Guard said the drone was shot down by its air force and its Third of Khordad air defense system. The guard said the drone fell in the Kouh-e Mobarak region in the Central district of Jask, some 750 miles southeast of Tehran, after the aircraft violated Iran’s airspace.

The two countries have escalated their rhetoric since two oil tankers were attacked in the Gulf of Oman last week. The United States blames Iran for the attacks; Tehran denies responsibility. 

The U.S. military previously accused Iran of firing a missile at another drone last week that was responding to the attack on the oil tankers. In recent weeks the U.S. has sent an aircraft carrier to the Mideast and deployed additional troops to join tens of thousands already in the region.

The U.S. and Iran: Iran says Trump playing ‘very dangerous game’ 

Relations between the U.S. and Iran have been steadily deteriorating since President Donald Trump pulled the U.S. out of a global nuclear deal with Tehran a year ago. The administration brought back sanctions on banking, oil and petrochemicals.

Other nations sought to keep the nuclear deal in place, and Iran was slow to formally bow out. However, this week Iran’s nuclear agency warned the country was 10 days away from breaking the uranium stockpile limit set under the agreement.

Sina Toossi, research associate at the National Iranian American Council, a Washington-based nonprofit supporting human rights and democracy for Iran, said Iran must be condemned by the international community if it is proven that the drone was shot down over international waters. But Toossi also accused John Boton, Trump’s national security adviser, for provoking Iran-U.S. tensions.

“There are no military solutions to the U.S. disputes with Iran, only diplomatic ones,” Toose said. “However, rather than pursue sincere diplomacy, President Trump has elected to pile on pressure with no strategic foresight at the behest of uber-hawkish advisors like John Bolton.”

Trump on Iran: President says war would lead to ‘end’ of Iran