While the United States was fixated on the second presidential debate on Sunday night, one of its ships was under fire in the Middle East. Details remain few, but reports suggest that the Houthis, a Yemeni militant group backed by Iran, fired two anti-ship missiles at the USS Mason as it patrolled the Bab el-Mandeb Strait. Fortunately, the two missiles failed to hit the destroyer, falling into the sea. However, the incident marks a major escalation in the Yemeni civil war and highlights the growing threat that Iran’s proxies pose to a waterway that accounts for 8 percent of global trade that transits the Red Sea.

The attack on the Mason comes the same day that the Houthis fired a ballistic missile into Saudi Arabia, reaching Taif, a major city just east of the holy city of Mecca. That attack, the furthest into Saudi territory since the Saudi-led coalition of Muslim states began its military campaign against the Houthis and their allies in March 2015, is believed to be a response to a coalition airstrike in Yemen’s capital, Sanaa, which killed 140 mourners attending a funeral on Saturday.

The 19-month-long conflict remains at a stalemate. The Saudi-led coalition and allied Yemeni forces have recaptured much of the south, but they have failed to retake Sanaa and the Red Sea coast. Meanwhile, the Houthis, with forces loyal to former Yemeni president Ali Abdullah Saleh, continue to launch sporadic rocket and missile attacks into Saudi Arabia. Indeed, the Houthis and Saleh’s forces have frustrated the better-armed coalition, prompting strikes that sometimes appear to lack a strategic rationale and even violate international laws.

The brutality and relentlessness of coalition strikes have prompted the Obama administration to announce that it would be reviewing its support for the Saudi-led operation. This comes on the heels of efforts in Congress last month to block further arm sales to Saudi Arabia. Until now, the U.S. has limited its role in the operation to providing aerial-refueling to coalition planes as well as intelligence support (American drones also continue to loiter over the country, as they have for more than a decade, hunting for local al-Qaeda operatives). Sunday’s attack on the Mason, however, may reverse any decision to abandon the Saudi coalition – and it might even prompt a direct response from the U.S.

While it may have seemed surprising, Sunday’s attack is not the first such incident in the strait. Just last weekend, the UAE-owned logistics ship Swift was struck and nearly destroyed by a Houthi missile as it transited the Bab el-Mandeb Strait. A year earlier, the Houthis claimed a similar attack, releasing a video showing what appeared to be an Emirati Baynunah-class warship as it patrolled along the Yemeni coast. While the Emiratis did not acknowledge the Baynunah incident, video and images of the Swift reveal extensive damage to the vessel. It remains unclear whether the missile used was an Iranian-supplied C-802 (Iranian copies are known as the Noor) or a C-801 that was supplied to Yemen by China in 1995, but it is worth noting that Iran has a record of supplying C-802 missiles to Hezbollah.

The Swift attack was what prompted the U.S. to deploy the Mason and two additional ships, the destroyer USS Nitze and the laser-equipped USS Ponce, to the strait. According to Pentagon sources, the shore-launched missiles that targeted the Mason Sunday required the warship to deploy “onboard defensive measures.” The Mason, an Arleigh Burke-class guided-missile destroyer, reportedly fired two SM-2 missiles, an Evolved Sea Sparrow missile, and a Nulka decoy to defend itself and the nearby Ponce against the first missile.

According to reports, the second Houthi missile was fired nearly 60 minutes after the first. Surprisingly, there is no indication that the Mason or any other nearby American warship returned fire on the missile launch sites. It remains a puzzle as to why the U.S. Navy dispatched ships to the strategic waterway if they are not prepared to neutralize such threats.

So far, the U.S. has only responded with strong words, stopping short of saying that the U.S. military is prepared to retaliate and preserve freedom of navigation in a key international shipping lane. While the Obama administration, particularly in its last three months, is likely not eager to be tempted into yet another conflict in the Middle East, it must find a way to ensure the security of the Bab el-Mandeb Strait and to send a message to the Houthis and their Iranian backers.

Patrick Megahan is a research analyst at the Foundation for Defense of Democracies focusing on military affairs. He manages the website militaryedge.org. Follow him on Twitter @PatMegahan.