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The battle for an ancient desert city in war-torn Yemen has become a key to understanding wider tensions now inflaming the Middle East and the challenges facing any efforts by to shift US troops out of the region.

Fighting has been raging in the mountains outside as Iranian-backed , who hold Yemen’s capital of Sanaa, attempt to seize the city, which is crucial to the country’s energy supplies.

, which has led a military coalition since 2015 backing Sanaa’s exiled government, has launched airstrike after airstrike to blunt the Houthi advance toward Marib. The Houthis have retaliated with drone and missile attacks deep inside Saudi Arabia, roiling global oil markets.

The battle for Marib likely will determine the outline of any political settlement in Yemen’s second civil war since the 1990s. If seized by the Houthis, the rebels can press that advantage in negotiations and even continue further south. If held, Yemen’s internationally recognised government saves perhaps its only stronghold as secessionists challenge its authority elsewhere.

The fight also squeezes a pressure point on the most powerful of America's Gulf Arab allies and ensnarls any US return to Iran’s nuclear deal. It even complicates efforts by Biden’s administration to slowly shift the longtime mass US military deployments to the Mideast to instead counter what it sees as the emerging threat of China and Russia.

Losing Marib would be “the final bullet in the head of the internationally recognised government,” said Abdulghani al-Iryani, a senior researcher at the Sanaa Center for Strategic Studies. “You’re looking at a generation of instability and humanitarian crisis. You also will look at a free-for-all theatre for regional meddling.”

Marib, 120 kilometres (75 miles) east of Yemen's capital, Sanaa, is now home to over 800,000 refugees fleeing the Houthis, according to the United Nations' refugee agency. The fighting disrupts their access to water, electricity, food and education for their children.

“It was once a rare place in Yemen that enjoyed a degree of security and stability,” said Mohsen Nasser Al-Mouradi, political activist living near the city. “Now we hear the sounds of heavy weapons all day. We are under constant siege.”

For a while, beginning in the fall of 2019, Saudi Arabia reached a detente with the Houthis, said Ahmed Nagi, a non-resident Yemen expert at the Carnegie Middle East Center. Citing two Houthi officials familiar with the discussions, Nagi said a back channel agreement saw both the Saudis and the rebels refrain from attacking populated areas.

But when the Houthis began to push again into Marib, the Saudis resumed a heavy bombing campaign.

For the Houthis, “they think they gain through war more than peace talks,” Nagi said. For the Saudis, who increasingly signal they want an end to the conflict, "if they lose Marib, they’ll have zero cards on the negotiating table.”

Biden early in his term announced the US would halt support for Saudi Arabia's offensive combat operations in Yemen, saying: “This war has to end.” He also removed the Houthis from a list of “foreign terrorist organisations.”

But fighting around Marib has only escalated. Iran’s frustration over the Biden administration’s failure to swiftly lift sanctions has contributed to “an intensification of attacks by groups in Iraq, and the same in Yemen,” said Aniseh Bassiri Tabrizi, an Iran scholar at Britain's Royal United Services Institute.

“Iran is trying to deliver a message to the US,” Tabrizi said, “a message that the status quo is not sustainable.”

While experts debate how much control Iran exerts over the Houthis, the rebels increasingly launch bomb-laden drones previously linked to Tehran deep inside the kingdom.

“The US administration’s removal of the Houthis from the (foreign terrorist organisation) list, unfortunately, appears to have been misinterpreted by the Houthis,” the Saudi government said in a statement to The Associated Press. “This misreading of the measure has led them, with support from the Iranian regime, to increase hostilities.”

Since the war began, the Houthis have launched over 550 bomb-laden drones and more than 350 ballistic missiles toward Saudi Arabia, the kingdom said. While that has caused damage, injuries and at least one death, the war in Yemen reportedly has seen over 130,000 people killed. Saudi Arabia repeatedly has been criticised internationally for airstrikes killing civilians and embargoes exacerbating hunger in a nation on the brink of famine.

Biden's efforts to end the US involvement in Yemen's war come as his administration attempts to re-enter Iran's nuclear deal with world powers. Indirect talks began Tuesday in Vienna.

“The Iranians are keen to trade in their Yemen card for something more durable,” said Al-Iryani, the Sanaa Center researcher.

Such a deal might suit American interests. Biden's Defense Department is conducting a renewed look at troop deployments, particularly those in the Mideast, amid what experts refer to as the “great powers conflict" America faces with China and Russia.

However, such moves likely will be easier said than done.

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US troops remain in Afghanistan, Iraq and Syria. Meanwhile, the Gulf Arab nations like Saudi Arabia rely on US forces stationed in their countries as a counterweight to Iran.

Overall, American forces will remain in the Mideast, which remains crucial to global energy markets and includes three major choke points at sea for trade worldwide, said Aaron Stein, the director of research at the Philadelphia-based Foreign Policy Research Institute. What those forces look like, however, will change as the US weighs how to approach China and Russia while still trying to counterbalance Iran through a return to the nuclear deal, he said.

“It doesn’t solve the Iranian issue," Stein said. "It puts us in a place to manage it, like we’re in hospice care.”

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