The Middle East war is widening as Yemen’s Iran-backed Houthi rebels launch a spate of attacks on United States’ allies and close yet another key shipping waterway.

One of the strongest Iran-backed militias in the region, the Houthis entered the war in March, a month after Israel and the US attacked Iran, by firing missiles towards Israel. Today, the group is trading fire with Yemen’s internationally recognized government, and striking Saudi Arabia after a four-year de facto ceasefire with the kingdom.

The escalation comes while there is shaky maritime security in both the Strait of Hormuz and the Red Sea, and as Iran and the US give conflicting accounts of whether direct talks to reopen the waterway were taking place.

The latest attacks also raise the threat of an outbreak of another full-scale war in Yemen. The last war, which began in 2014, left the country facing the world’s worst humanitarian crisis.

Here’s what we know about the escalation and how it might impact the US-Iran war.