The United States launched airstrikes on Iraqi and Syrian targets linked to Iran's Revolutionary Guard and the militias Tehran backs, following an attack on one of the many sites across the Middle East where the US has a military presence.
Here is what we know about the US military presence in the Middle East:
WHERE ARE US BASES IN THE MIDDLE EAST
The US has operated bases around the Middle East for decades. At its peak, there were more than 100,000 US troops in Afghanistan in 2011 and over 160,000 personnel in Iraq in 2007.
While the number is far lower after withdrawal from Afghanistan in 2021, there are still about 30,000 US troops scattered across the region. In addition, since the Israel-Hamas war began in October, the US has temporarily sent thousands of additional troops to the region, including on warships.
The largest US base in the Middle East is located in Qatar, known as Al Udeid Air Base and built in 1996. Other countries where the US has a presence include Bahrain, Kuwait, Saudi Arabia and the United Arab Emirates (UAE).
The US has roughly 900 troops in Syria, in small bases like al Omar Oil field and al-Shaddadi mostly in the northeast of the country. There is a small outpost near the county's border with Iraq and Jordan, known as the Al Tanf garrison.
There are 2,500 personnel in Iraq, spread around facilities like Union III and Ain al-Asad air base, though talks are ongoing about the future of those troops.
WHY ARE US TROOPS STATIONED IN THE REGION?
US troops are stationed in the Middle East for different reasons and with the exception of Syria, they are there with the permission of each country's government.
In some countries like Iraq and Syria, US troops are there to fight against Islamic State militants and are also helping advise local forces. But they have come under attack from Iran-backed forces over the past several years and have taken action against them.
Jordan, a key US ally in the region, has hundreds of US trainers and they hold extensive exercises throughout the year.
In other cases, like in Qatar and the UAE, US troops have a presence to reassure allies, carry out training and are used as needed in operations in the region.
ARE THERE FOREIGN MILITARY BASES IN THE US?
While Washington's allies sometimes send their troops to train or work with US troops, there are no foreign military bases inside the US.
TOWER 22 IN JORDAN
Tower 22, a base in Jordan at the most northeastern point where the country's borders meet Syria and Iraq, was hit in a drone attack on Jan 28 that killed three Army Reserve soldiers. Washington blamed Iran-backed, Iraqi militia Kataib Hezbollah.
Specifically, Tower 22 is near Al Tanf garrison, which is located across the border in Syria, and which houses a small number of US troops. Tanf had been key in the fight against Islamic State and has assumed a role as part of a US strategy to contain Iran's military build-up in eastern Syria.
DO US BASES IN THE REGION GET ATTACKED OFTEN?
US bases are highly guarded facilities, including with air defence systems to protect against missiles or drones. Facilities in countries like Qatar, Bahrain, Saudi Arabia, Kuwait are not usually attacked.
But US troops in Iraq and Syria have come under frequent attack in recent years. Since Oct 7, US troops have been attacked more than 160 times by Iran-backed militia, injuring about 80 troops, even prior to Sunday's attack on Tower 22, which injured around 40 more.
A new wave of violence in the Middle East erupted after Oct 7 when Islamist Palestinian Hamas fighters burst into Israel and killed 1,200 Israelis and took 253 others hostage. In response, Israel unleashed a military campaign that has killed more than 27,000 people, according to Gaza health authorities, and caused a severe humanitarian crisis in the densely populated Gaza Strip.