US-Iran tensions: A timeline of key events

President Trump piled sanctions on Iran after military officials said the country downed an unmanned U.S. drone last week. The U.S. response comes after the White House and Pentagon were considering military strikes against the regime – but called them off once Trump learned up to 150 Iranians could be killed.

Tensions between the U.S. and Iran have been brewing since Trump withdrew from the Iran Nuclear Deal on May 8, 2018. Punishing sanctions have been levied against Iran’s economy in the months since.

WHAT IS THE STRAIT OF HORMUZ, WHERE IRAN SHOT DOWN US NAVY DRONE?

Here’s a timeline of what led to the downing of the drone and the U.S. response.

April 8

The U.S. designated Iran’s Islamic Revolutionary Guard Corps, a division of the Iranian military, a foreign terrorist organization. It was the first time the U.S. placed part of a foreign government under such a classification. Iran retaliated by designating the U.S. military as terrorist and labeling the U.S. a "supporter of terrorism."

Iran's Supreme Leader Ayatollah Ali Khamenei. (Office of the Iranian Supreme Leader via AP)

Iran's Supreme Leader Ayatollah Ali Khamenei. (Office of the Iranian Supreme Leader via AP)

May 2

The U.S. ended waivers that allow other countries to import Iranian oil in an effort to reduce its oil exports to zero and put the regime under unprecedented economic pressure. Countries that had been issued waivers, like China, Japan, India and others, would face U.S. sanctions if they continued to import oil from Iran.

May 5

White House national security adviser John Bolton announced the U.S. was deploying a U.S. aircraft carrier strike group and a bomber task force to the Middle East following “clear indications” that Iranian and Iranian proxy forces were preparing for a possible attack on American troops in the region.

Bolton reiterated that the U.S. is not seeking war with Iran, “but we are fully prepared to respond to any attack, whether by proxy, the Islamic Revolutionary Guard Corps, or regular Iranian forces.”

May 8

Iran's President Hassan Rouhani. (Iranian Presidency Office via AP)

Iran's President Hassan Rouhani. (Iranian Presidency Office via AP)

The Trump administration squeezed Iran with additional sanctions on its steel, aluminum, copper and iron industries.

Tehran announced its withdrawal from parts of the 2015 nuclear deal if it didn't receive economic relief and threatened to enrich its uranium stockpile closer to weapons-grade levels in 60 days.

May 24

Trump announced an additional 1,500 troops will be sent to the Middle East amid growing tensions with Iran. He said the troops were to serve in a “mostly protective” role.

June 13

Two oil tankers were attacked in the Gulf of Oman near the strategic Strait of Hormuz. Secretary of State Mike Pompeo blamed Iran for what he called a "blatant assault." Iran denied any involvement in the attacks and accused Washington of waging an "Iranophobic campaign" of economic warfare.

"Iran stands ready to play an active and constructive role in ensuring the security of strategic maritime passages as well as promoting peace, stability and security in the region," Iran's U.N. Mission replied in a statement at the time.

June 17

Iran announced it was 10 days away from exceeding the low-enriched uranium stockpile limit it agreed to in the 2015 nuke deal.

The Pentagon later approved sending approximately 1,000 additional troops to the Middle East "to address air, naval, and ground-based threats" in the region, according to then-Acting Defense Secretary Patrick Shanahan.

June 20

Iran shot down a U.S. Navy drone over the Strait of Hormuz. U.S. military officials called the strike an “unprovoked attack” in international airspace, while Iranian officials disagreed, claiming the drone had “violated” its airspace.

June 21

Trump confirmed he called off a retaliatory strike against Iran in response to the downing of the U.S. drone. He said he canceled it 10 minutes before the attack, saying the number of expected casualties was not "proportionate" to what Tehran did.

June 24

Trump issued “hard-hitting” financial sanctions against Iran’s Supreme Leader Ayatollah Ali Khamenei and his associates. He said the sanctions came in response to the regime’s recent aggressive actions, which included shooting down a U.S. drone.

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Fox News confirmed the U.S. military also carried out a cyberattack against Iran on June 20 following the downing of the American drone. Sources said U.S. Cyber Command launched the cyberattack targeting the Iranian intelligence and radar installations used to down the U.S. Navy drone.

Fox News' Andrew O'Reilly, Alex Pappas and Greg Norman and The Associated Press contributed to this report.