The US military said that it sunk three Houthi vessels in the Red Sea in a statement on Sunday, and killed 10 fighters of the Yemeni armed group after a clash in the middle of one of the world’s most important trade waterways.

The escalation follows weeks of Houthi attacks on ships passing through the sea, ostensibly in a bid to pressure Israel to stop its devastating war on Gaza that has killed more than 21,000 Palestinians in the besieged strip.

US destroyers have teamed up with a few other nations to try and stop the Houthi attacks in the Red Sea under Operation Prosperity Guardian — though several of its partners have distanced themselves from the initiative.

The Red Sea clash on Monday was the first major direct military engagement between the US military and Houthi fighters. Here is what we know about how it all unfolded:

What happened on Sunday?

On Sunday at 6:30am Yemen time (03:30 GMT), the Denmark-owned and Singapore-flagged container ship Maersk Hangzhou issued a second distress call in a day, reporting being attacked by four “Houthi small boats”, the US Central Command (CENTCOM) posted on X. CENTCOM added that the Houthis tried to board the Maersk vessel that was sailing through the Southern Red Sea.

In response to this distress call, helicopters from two warships, the USS Eisenhower and USS Gravely, flew towards the Maersk ship. CENTCOM said the crew of the USS Gravely destroyer first shot down two antiship ballistic missiles fired at the Maersk vessel.

Why did the US sink Houthi boats?

Four small boats then attacked the same cargo ship with small arms fire and Houthi fighters tried to board the vessel, the US Navy said. The US Navy said it then warned the fighters to stay away from the ship, at which point, the helicopters came under fire.

The helicopters fired back, sinking three of the boats. Houthi crew members in the boats were killed. The fourth boat escaped the area and US personnel and equipment did not bear harm, CENTCOM added.

“We’re going to act in self-defence going forward,” a White House official said.

A spokesman for the Houthis confirmed that ten of their fighters were “dead and missing” after their boats were attacked.

How have Maersk and other shipping firms reacted?

Maersk announced on Sunday that it was pausing all sailing through the Red Sea for the next 48 hours. On December 27, it had scheduled several dozen container vessels to travel via the Suez Canal and the Red Sea in the coming days and weeks.

Together with German shipping company Hapag-Lloyd, Maersk operates almost a quarter of the world’s shipping fleet.

Other shipping firms have also responded to the escalating maritime conflict. Firms including CH Robinson, Evergreen, HMM, Ocean Network Express, Wallenius Wilhelmsen and Yang Ming Marine Transport are planning to avoid the Red Sea and increase the number of ships rerouting around Africa’s Cape of Good Hope rather than the Suez Canal. This new route is longer and costlier. Evergreen has also temporarily stopped accepting Israeli cargo.

While these attacks have had a “fairly limited” impact on the oil market so far, experts postulate that prices could rise if the situation continues.

Why are the Houthis attacking ships in the Red Sea?

The Iran-backed Houthis began launching drones and missiles towards the southern parts of Israel in October soon after the war broke out on October 7.

However, the drones were intercepted or fell short. In a change of tactic, the group started attacking ships in the Red Sea that they claimed were linked to Israel. Their attacks have disrupted many ships for making their way into Israel.

On December 19, US Defense Secretary Lloyd Austin announced the establishment of a multinational maritime protection force to counter the Houthis. Despite this, the Houthis have said they will not back down unless Israel ceases its war on Gaza.

Despite a multinational coalition agreeing to support the US cause, only the United Kingdom has directly contributed warships, leaving Washington to effectively “act alone” against the Houthis, reports Al Jazeera’s Resul Serdar from Djibouti on the edge of the Red Sea.

Serdar added that the US has “not been able to deter the Houthis” so far, with the group waging attacks even more frequently. He added that the latest clash marked a serious escalation because the US not only sunk Houthi boats but also killed Houthi fighters. Such confrontations are sparking fears of a regional escalation of the war that could pull Yemen into the conflict.