Iran-backed Houthi forces have seized a small island most Americans have never heard of, but its location could affect what they pay for gasoline, diesel and goods shipped from around the world.

The Houthis took control of Perim Island, also known as Mayyun, in Yemen’s Bab al-Mandab Strait on Friday, according to Yemeni government officials. The island sits directly in the narrow waterway connecting the Red Sea with the Gulf of Aden and the Indian Ocean.

The capture came a day after the Houthis seized the Yemeni port city of Mokha and continued their drive south along the Red Sea coast.

Perim is the bigger prize.

The island divides the Bab al-Mandab into two shipping channels. Ships traveling between the Mediterranean and the Indian Ocean through the Suez Canal must pass through the Red Sea and then the Bab al-Mandab.

That includes oil tankers, container ships carrying consumer goods and food, and U.S. Navy warships moving between the Mediterranean, Middle East and Indian Ocean.

About 12 percent of world trade normally moves through the Bab al-Mandab route. An estimated 7 percent of the world’s oil production passes through the strait.

The Houthi advance comes at an especially dangerous time.

Shipping through the Strait of Hormuz, on the other side of the Arabian Peninsula, has already been severely disrupted by the war with Iran. Before the war, about 125 large commercial vessels a day passed through Hormuz. On Thursday, Reuters tracking data showed only seven.

That made the Red Sea increasingly important as an alternative route for Saudi Arabian oil and other shipments trying to avoid Hormuz.

Now that alternative is threatened as well.

The Houthis already controlled much of Yemen’s Red Sea coast farther north. With the capture of Mokha, the coastal town of Dhubab and Perim Island, they have extended their reach all the way to the Bab al-Mandab itself.

The oil market has already reacted.

Brent crude, the international benchmark, climbed above $107 a barrel Thursday before easing Friday morning to about $104. Brent and U.S. crude were still headed toward weekly gains of more than 8 percent as traders worried about Middle Eastern supplies and the safety of the shipping routes used to move them.

Higher crude prices do not appear instantly on the neighborhood gas-station sign, but sustained increases eventually work through refineries, wholesalers and retailers. Diesel prices also affect trucking and shipping costs, which can eventually raise the price of everything from groceries to manufactured goods.

The problem extends far beyond oil.

If shipping companies decide the Bab al-Mandab is too dangerous, vessels traveling between Asia and Europe can avoid the Red Sea and Suez Canal by sailing around the southern tip of Africa. That adds considerable distance, time and fuel to a voyage.

The world has seen this before. Earlier Houthi attacks on Red Sea shipping forced major shipping companies to divert vessels around Africa, increasing freight costs and transit times.

The United States is already involved in the widening fight.

More than 100 U.S. military advisers are in Saudi Arabia providing intelligence and assistance identifying Houthi targets, CNN reported Thursday. One American official estimated that roughly 200 U.S. personnel are involved in a recently created joint command.

The United States is sharing intelligence and helping Saudi forces locate potential targets, but American forces are not currently carrying out the strikes themselves, according to CNN.

Saudi Arabia responded to the latest Houthi advance Friday with airstrikes against the airport at Mokha, according to Houthi media. There were no immediate reports of casualties.

For the U.S. Navy, the geography creates another problem — where to find the ships to deal with it.

American warships regularly use the Red Sea and Bab al-Mandab when moving between the Mediterranean and waters around the Arabian Peninsula and Indian Ocean. The Navy has also spent years defending ships in the Red Sea against Houthi missiles and drones.

Now the Houthis have a position directly inside the waterway.

Control of Perim does not mean the Houthis can simply close the Bab al-Mandab. Commercial and military vessels were still moving through the strait Thursday. But possession of the island could give the Houthis another location from which to watch shipping and potentially threaten vessels passing through the narrow waterway.

Keeping that route open could require additional U.S. Navy ships and aircraft at a particularly difficult time.

The Houthi advance also puts them across a narrow stretch of water from one of America’s most important military installations in Africa.

Camp Lemonnier, the U.S. military’s only enduring base on the African continent, is located in Djibouti on the opposite side of the Bab al-Mandab. The Navy says the base supports approximately 4,000 U.S., allied and partner military and civilian personnel and contractors and serves as the primary base for American operations in the Horn of Africa.

The geography is difficult to ignore.

The Bab al-Mandab is only about 18 miles wide at its narrowest point. Perim Island sits inside the strait and divides it into two shipping channels. On one side is Yemen, where Houthi forces have now seized Perim and advanced to the water’s edge. On the other is Djibouti and a permanent American military presence.

Camp Lemonnier supports U.S. aircraft, ships and operations throughout the region. That means the worsening fight around the Bab al-Mandab is not occurring in some distant corner of Yemen. It is unfolding along the maritime approaches to a major American base.

That could add another demand on U.S. forces already stretched by the war with Iran, protecting shipping and maintaining American commitments elsewhere in the world.

The Navy is already stretched across the Middle East and other parts of the world. The war with Iran has required ships and aircraft around the Persian Gulf, Arabian Sea and Strait of Hormuz. At the same time, the Navy must maintain forces in the Mediterranean and Pacific and meet its other commitments around the globe.

The strain has already shown.

The USS Abraham Lincoln and its crew spent more than nine months deployed, much of that time without a port visit, before the USS George Washington carrier group arrived in the Middle East to help relieve it. Ships supporting those carriers also face long deployments, maintenance demands and the constant need for fuel, ammunition, food and spare parts.

The Navy cannot create another destroyer because another shipping lane suddenly becomes dangerous.

If protecting Red Sea traffic again requires a larger American naval presence, those ships have to come from somewhere. Sending additional destroyers to Bab al-Mandab could mean extending deployments, delaying maintenance or reducing the number of ships available somewhere else.

That is what makes the Houthi advance more than another development in Yemen.

The United States could soon face two dangerous shipping chokepoints at the same time: the Strait of Hormuz on one side of the Arabian Peninsula and Bab al-Mandab on the other.

The United States is already fighting Iran while trying to restore traffic through Hormuz. It is helping Saudi Arabia fight the Iran-backed Houthis. And now the Houthis have moved into one of the world’s other critical shipping chokepoints.

For American consumers, the immediate warning is in the price of oil and the cost of moving goods.

For the Navy, the problem is simpler and harder to solve.

There are only so many ships.

American ships regularly use the Red Sea and Bab al-Mandab when moving between the Mediterranean and waters around the Arabian Peninsula and Indian Ocean. The Navy has spent years dealing with Houthi missiles and drones threatening vessels in and around the Red Sea.

Control of Perim does not mean the Houthis can simply close the Bab al-Mandab. Commercial and military vessels were still moving through the strait Thursday, when tracking data recorded 26 commodity vessels passing through, close to the recent daily average.

But the island gives the Houthis a position directly inside the waterway.

That raises the risk to every vessel passing it and gives shipping companies another reason to reconsider whether the shorter Red Sea route is worth the danger.

It also presents Washington with an increasingly difficult problem.

The United States is already fighting Iran while trying to restore traffic through the Strait of Hormuz. It is helping Saudi Arabia fight the Iran-backed Houthis. And now the Houthis have moved into one of the world’s other critical shipping chokepoints.

The two waterways sit on opposite sides of the Arabian Peninsula.

Hormuz is the front door for much of the Persian Gulf’s oil.

Bab al-Mandab has increasingly become the way around it.

Now both doors are in danger.