Iran-backed Houthi militants in Yemen used advanced missiles and drones to target the United Arab Emirates on Monday, according to people briefed on the investigation—a deadly demonstration of the expanding threat to Middle East security posed by Tehran’s emboldened allies.

The Houthi strike amounted to the most visible display of the military advancements made by a militia that less than a decade ago relied on machine guns and rocket launchers but now can help Iran project power almost 1,000 miles away from their Yemeni mountain strongholds. The Houthis have hit the U.A.E. before, but this was the first time the Emiratis acknowledged it.

Monday’s attack was the latest in a series of provocative moves the Houthis have taken in recent months that have raised alarms from Washington to Riyadh.

Earlier this month, the Houthis seized an Emirati-flagged ship off the Yemen coast that they have refused to release. In November, the militant force took control of the shuttered American Embassy in San’a, the Yemeni capital, and has rebuffed U.S. demands that they free several Yemenis who worked for the U.S. The Houthis have also detained two United Nations workers for more than two months.

With its strikes on Abu Dhabi’s airport and a state-owned oil facility, the Houthis were ostensibly retaliating for the U.A.E.’s intensified role supporting the Saudi-led coalition in the Yemen civil war, which resulted in the Houthis’ first significant setback last week after years of territorial gains. Three people were killed in Abu Dhabi, prompting Saudi coalition airstrikes that killed 11 people in Yemen, including at least five civilians, the U.N. said.

Beyond the battlefield, the Houthi ability to strike in the heart of rival capitals in Saudi Arabia and the U.A.E. gives Tehran leverage in its nuclear-containment talks in Vienna and is one of the forces driving a larger geopolitical realignment in the Middle East in recent months.