The maritime artery, only 18 miles wide at its narrowest point, joins the Red Sea and the Gulf of Aden — and by extension, the Mediterranean Sea and the Indian Ocean. It is a vital energy pathway, with 4 million barrels of oil and petroleum products a day passing through on tankers heading to or from the Suez Canal, nearly 1,400 miles to the north.

All parties to the Yemen conflict, and many looking on looking on from near and far, are keenly aware of the strait's significance as a potential choke point. Even before the current fighting erupted, Egypt — for which the Suez Canal is a crucial source of revenue — threatened to intervene militarily if the Bab al Mandab were threatened.

Iran acknowledged this week that it had sent two warships, the Alborz and the Bushehr, to the mouth of the strait, describing the deployment as intended to protect commercial shipping against piracy, as its navy has done for years. But the move coincides with increasingly strident denunciations from Shiite Muslim Iran of the Saudi Arabia-led air campaign against Shiite Houthi insurgents and their allies, which began March 26 and has killed more than 1,200 people in Yemen.

Here are some questions and answers about the Bab al Mandab and its role in the conflict:

Why is the strait so important?

The opening of the Suez Canal in 1869 was a boon to trade and shipping, meaning that vessels sailing between Asia and the West no longer had to make the expensive and time-consuming voyage around the southern end of Africa. But to get to and from the canal, ships must pass through the strait, between Yemen and the tiny Horn of Africa nation of Djibouti.

What are its dimensions?

The Bab al Mandab is divided into two channels by the island of Perim, also called Mayyun in Arabic. The eastern channel, closer to Yemen, is the narrower one, only two miles wide, making it tricky for tankers to negotiate. The western channel is 16 miles wide. The name of the strait, long chronicled in historical accounts, is thought to refer to its navigational hazards.

Has it been used for blockades?

Yes, in 1973 Egypt used it to impose a naval blockade of Israel, which has access to the Red Sea through the Gulf of Aqaba.

Is the U.S. helping to secure the strait?

The Pentagon has pledged to ensure that the waterway remains open to shipping. U.S. Army Gen. Lloyd Austin, the head of U.S. forces in the region, said at a Senate hearing this week that the American military would work with Persian Gulf allies to protect the "free flow of commerce" through both the Bab al Mandab and the Strait of Hormuz, which connects the Arabian Sea and the Persian Gulf.

An American aircraft carrier, the Theodore Roosevelt, and a guided-missile cruiser were deployed last month near Yemen, but have since turned back to the Persian Gulf. Seven U.S. warships remain in the Gulf of Aden, military officials have said.

What are other countries doing?

The Saudi-led coalition has imposed a naval blockade on Yemen, saying its goal is to block the flow of arms from Iran to the Houthis, weaponry that Iran denies supplying. But the blockade has also caused widespread civilian suffering as food and medical supplies are depleted.

Egypt, which has sent warships to the area, says navigation of the Bab al Mandab and Red Sea is vital to its national security and that protecting it is a top priority. "We are a nation in danger, a nation which is defending and protecting itself," President Abdel Fattah Sisi recently told senior military officials.

Are the Houthi rebels in a position to threaten the Bab al Mandab?

So far, the insurgents have not interfered with shipping, but they hold key territory that could allow them to do so. Days after the start of the air campaign, Houthis entered a coastal military base overlooking the strait, in the Dabab district of Taizz province. A Yemeni intelligence official said troops and commanders were loyal to former strongman Ali Abdullah Saleh, who has allied himself with the Houthis, and opened the gates to the insurgents. The Saudi-led coalition has staged airstrikes on the military camp but has not succeed in driving out the Houthis or their allies.

Houthis have also based missiles, fast boats and other armaments on Perim Island in the strait, according to officials in Djibouti quoted by the Bloomberg news agency. The Yemen military is also deployed in the strait, but the intelligence official said the forces are Saleh loyalists who back the rebels. Houthis are also active in the Red Sea port of Hudaida, which lies close to the strait's entrance.

Even before the Saudi-led offensive began, some analysts were declaring that the Houthis considered the waterway a prime prize. In February, a Yemeni retired brigadier general, Mohsen Khasrouf, told the pan-Arab daily Asharq al Awsat that the insurgents had already made "the first steps on the road to taking control" of the Bab al Mandab.

But there are also indications that the rebels are aware that any overt threat to navigation would be likely to draw a wider — and harsher — international response than the Saudi-led campaign. In addition, the Houthis have focused attention of late in the north, along the border with Saudi Arabia, with dozens of insurgents reported killed in an attack along the frontier near the town of Najiran, the largest such strike since the air assault began.

Special correspondent Zaid al-Alayaa in Sana, Yemen, contributed to this report.