The U.S. military directly attacked Houthi rebels in Yemen for the first time on Wednesday — firing Tomahawk cruise missiles at three rebel-held radar stations on the Red Sea coast. The attack, which was in retaliation for a failed missile attack on a U.S. Navy destroyer on Sunday, risks drawing the U.S. further into the 18-month war.

In March 2015, a coalition of states led by Saudi Arabia began a U.S.-backed bombing campaign against the Houthi forces, which four months earlier had seized Yemen’s capital and deposed the country’s U.S.- and Saudi-backed dictator. Since then, the U.S. has flown refueling missions for Saudi aircraft, supplied targeting intelligence, and resupplied the Saudi effort with tens of billions of dollars of weapons.

While the U.S. has previously conducted direct attacks in Yemen against al Qaeda — which controls vast territory in central and eastern Yemen — it had not directly engaged Houthi forces before.

The escalation began last week when the U.S. dispatched warships to the Bab al-Mandab Strait — which connects the Red Sea to the Gulf of Aden — after the Houthis fired on and nearly sank a ship from the United Arab Emirates. The UAE is a part of the Saudi-led bombing coalition, which has maintained a strict naval blockade of the country since the war began.

When the Houthis fired on the U.S.S. Mason earlier this week, sailors were able to deploy countermeasures and the ship was not damaged.

The Department of Defense issued a statement describing the U.S. attack as a series of “limited self-defense strikes,” but promised to “respond to any further threat” to U.S. ships “as appropriate.”

“The intent of our strikes were to deter future attacks and to reduce the risk to U.S. and other vessels,” White House Deputy Press Secretary Eric Schultz said Thursday. “We are prepared to respond if necessary to any future missile launches.”

The U.S. Navy tweeted a video of the destroyer U.S.S. Nitze launching cruise missiles, captioning it with the hashtag “#Yemen” — commonly used by activists to draw attention to the humanitarian catastrophe.