For more than a year, the Houthis, an Iranian-backed militant group that controls northern Yemen, have attacked ships in the Red Sea, severely disrupting a major trade route, and have targeted Israel with drones and missiles.

This month, the pace of Houthi attacks on Israel has increased — as have Israeli retaliations.

Over the past week, the Houthis said they had launched several attacks on Israel.

In response, the Israeli military on Thursday bombed parts of Yemen, including the international airport in the capital, Sana, leaving four people dead according to the local health ministry. It was Israel’s fourth attack in Yemen over the past year and came just a week after Israel last struck.

The United States and Britain have also retaliated against the Houthis to protect international waterways. The U.S. military on Saturday said that it had targeted Houthi facilities in Yemen.

Here’s what to know about the Houthis, their attacks on ships and their conflict with Israel.

Who are the Houthis?

The Houthis, Shiite militants who have been fighting Yemen’s government for about two decades, overran the Yemeni capital, Sana, in 2014, forcing the country’s internationally recognized government to flee to the southern city of Aden.

A Saudi-led coalition launched a military intervention to oust the militants but failed, leaving the Houthis in power in northern Yemen, ruling most of the population and igniting a civil war that has killed hundreds of thousands and led to one of the world’s worst humanitarian crises.