— -- Yesterday, the U.S. military fired Tomahawk missiles at radar sites operated by Houthi rebels inside Yemen, retaliation for the failed targeting of two U.S. Navy ships by cruise missiles earlier this week.
It is the first time the U.S. has gotten directly involved militarily in Yemen’s internal fighting between the Saudi-backed government and the Houthis, a Shiite militant tribal group backed by Iran that seized power in January 2015.
ABC News looks at the complex role the U.S. has played during this bloody war.
U.S. Support of Saudi Arabia is Tested
Shortly after the beginning of the war in January 2015, regional powers rushed in to back the various parties in their internal struggle. Iran began to provide military support to the Houthis. In March, Saudi Arabia and other Gulf nations intervened military to restore the government back to power.
The United States, a longtime ally of Saudi Arabia, quietly began supporting the Saudi-led coalition by providing it with intelligence, weapons sales and air-refueling support.
But the U.S. support of the Saudis has been tested.
A Saudi airstrike on a funeral hall on Saturday may have killed more than 100 civilians alone. A White House statement later said the administration was “deeply disturbed” by the airstrike and announced it was reviewing its limited military support for the coalition, warning that it was “not a blank check.”
The airstrike is the latest in a series of incidents that have raised the Obama administration’s concerns about how the fight against the Houthis is being waged. The Saudi coalition has waged an airstrike campaign that by some estimates has led to 2,000 civilian deaths.
Recently, the U.S. had one military adviser working with the coalition in its military planning in Yemen and has relocated to Bahrain the 45 personnel that had been doing that work at a military base in Saudi Arabia.
The U.S. Targeting of al-Qaeda in Yemen
Prior to the start of the civil war in 2015, the U.S. had 100 military advisers in Yemen supporting that government’s fight against al-Qaeda in the Arabian Peninsula (AQAP). As the war progressed, by February 2015, the U.S. was forced to suspend operations and its embassy in Yemen and evacuate the advisers out of concern for their safety.
AQAP, al-Qaeda’s most dangerous affiliate, has taken advantage of the power vacuum in Yemen to rebuild its operations. The group remains committed to taking over control of the country and carrying out terror attacks against the U.S. and other western countries, as well as threatening Saudi Arabia and other Gulf nations.
Fearing the security vacuum that AQAP is trying to exploit, the U.S. military continues to launch drone attacks against AQAP from a base in nearby Djibouti.
The U.S. military has launched 22 strikes since February of this year and estimates that those strikes have killed more than 100 al-Qaeda operatives, according to U.S. Central Command and Department of Defense press releases.
Other U.S. Interests in Yemen
Yemen is an impoverished country on the southwestern tip of the Arabian Peninsula next to Bab-el-Mandeb, a narrow strait separating Yemen and the Horn of Africa that joins the Red Sea and the Indian Ocean.
The U.S. has a strong interest in ensuring the safety of commercial shipping and the 3-to-4 million barrels of oil that travel through the vital waterway each day.
U.S. Navy ships have often been deployed to the waters off Yemen to prevent a military or terrorist threat to the key shipping lane.
A stable government in Yemen is important to Gulf countries wary of Iran’s military support and influence with the Houthis. Gulf countries fear an Iranian military presence in the Arabian Peninsula, a concern that would exacerbate the already tense relationship with Iran because of the threat its nuclear program has posed to the region.
The U.S. has also contributed more than $327 million in humanitarian assistance to Yemen in fiscal year 2016, according to the State Department. State estimates that over 3.1 million Yemenis have been displaced. Twenty-one million people, approximately 80% of the country, requires humanitarian assistance.
In the nearly two years of conflict, the United Nations estimates that at least 10,000 people have been killed.
On Sept. 27, the State Department responded to reports of U.S. citizens being held in Houthi-controlled territory, saying they were monitoring the situation and calling on the Houthis to release any U.S. citizens.
In May 2015, the Houthis held at least four U.S. citizens prisoner. One of those Americans, Casey Coombs, a freelance journalist, was released with the help of the government of Oman two weeks later.
In September 2015, two more Americans were freed, along with a British citizen and three Saudis. Those Americans, Scott Darden and Sam Farran, were working to get relief supplies into Yemen and as a security expert, respectively, according to the New York Times.
Three other Americans were freed by the Houthis in November 2015.
U.S. officials say that Yemen still has a small number of Americans in detention.
What’s Next for the U.S. in Yemen?
Three U.S. Navy ships are now in the southern Red Sea following the Houthi’s provocations against American military vessels. They include the destroyers USS Mason and USS Nitze as well as the USS Ponce, an afloat forward staging area that normally houses special operations forces.
The Houthi's actions highlight the long-range missile threat they can pose to neighboring countries and U.S. warships in the waters off Yemen. It is unknown if the missiles were Soviet-era missiles they took over from government forces or if they were new missiles that Saudi Arabia claims have been provided by Iran to the Houthis.
The U.S. has said it will retaliate if its ships are targeted again.
“The United States will respond to any further threat to our ships and commercial traffic, as appropriate, and will continue to maintain our freedom of navigation in the Red Sea, the Bab al-Mandeb, and elsewhere around the world,” Pentagon Press Secretary Peter Cook said in a statement on Wednesday.
But it’s unlikely the U.S. will seek broader military involvement with the Houthis in Yemen. Yesterday’s U.S. strike was retaliatory in nature and not necessarily indicative of a broader military operation in the country.
The White House seems content with allowing and enabling the Saudis to continue to lead this fight.
“We do share some intelligence with [the Saudis] but the United States does not do targeting for them,” Josh Earnest, White House press secretary, said yesterday.
ABC News' Justin Fishel contributed to this report.