Enemy bullets ripped overhead. A barefoot Yemeni soldier who calls himself Fouad the Brave grabbed a rifle and returned fire from behind a sand berm, taking aim at Iran-backed Houthi fighters a few hundred yards away.

The small desert outpost manned by Fouad and a handful of sunburned soldiers is on the front line of Yemen’s civil war, which pits government forces backed by Saudi Arabia and the United Arab Emirates against Houthi rebels allied with Tehran.

Marib is one of the last major sanctuaries the Yemeni government has in the north of the country. “Either we win,” Fouad said, “or we die trying.”

Saudi Arabia and its allies, with their local proxies trying to hold their ground and with Washington having scaled back support for the conflict, are struggling to turn the tide here, stepping up aerial bombing and missile strikes.

The Saudi-led coalition carried out roughly 700 airstrikes in February, according to the Yemen Data Project, a nonprofit that tracks the war in Yemen. That would make it the most intense period of bombing since 2018.

In the past four months, more than 1,500 Yemeni civilians have been killed or wounded, up from 823 in the previous four months, according to the Civilian Impact Monitoring Project, which collects information on the war for the United Nations. Saudi-coalition airstrikes were responsible for the vast majority of the casualties, the group said.

A central aim of the airstrikes, Saudi officials say: push the Houthis back and hurt them enough that they feel compelled to negotiate an end to the war. Peace talks have been stalled for months as the Houthis try to capture Marib.

The U.S. and U.N. have urged Saudi Arabia to ease up on airstrikes. But officials in Riyadh and Yemen say they intend to hit the Houthis even harder.

“We have to continue the fight,” said Sheikh Sultan al-Aradah, the governor of Marib. His home was destroyed in September by two Houthi ballistic missiles. “This is the right track, but it’s just the beginning.”

The Houthis have responded to the Saudi and U.A.E. moves by launching missile and drone strikes targeting the Gulf nations. They have also fired more missiles at Marib, including a barrage of seven that crashed into the city on Feb. 19 while a Wall Street Journal reporter and photographer were visiting.

The intensifying violence comes seven years after Saudi Arabia and a small group of allied nations launched a bombing campaign that Riyadh said would take only a few weeks to rout Houthi fighters who had taken Yemen’s capital, San’a, in a conflict that grew out of the Arab Spring.

Instead, the war has dragged on and created one of the worst humanitarian crises in the world, with the U.N. estimating that more than 377,000 people have died as a result of the war, 70% of them children.

The war in Yemen also gave Iran an opening to transform outgunned Houthi fighters into one of Tehran’s most adept militant allies. The group can now fly advanced drones and fire long-range missiles capable of hitting the capitals of Saudi Arabia and the U.A.E. more than 800 miles away.

While Saudi-backed fighters say they have made some gains in recent weeks, the Houthis still control the country’s capital and much of the nation’s northern highlands. The fractured Yemeni government and its allies retain fragile control in the south and east.

If the Houthis take Marib, it would give them effective control of the entire north of Yemen, along with oil money they can use to keep financing their fight.

“If they control Marib, we will lose the war and lose security and stability in the region,” said one top Saudi official.

“My weapon was a pen, but now it’s a gun,” Mr. Adel said from his hospital bed in between taking gasps of oxygen from his mask after being shot by Houthis. “They forced me to fight.”

The U.S. and Saudi Arabia have accused Iran of providing the Houthis with weapons, advisers and support they have used to build and launch an expanding array of drones and missiles targeting Riyadh, Abu Dhabi and ships off the Yemeni coast.

Iran is one of the few countries to have diplomatic relations with the Houthis. Tehran has denied that it provides them with weapons.

The Houthis are focusing on Marib in an effort to deliver a debilitating blow to the Saudi-backed government. Saudi officials say the Houthis are refusing to negotiate while they try to seize Marib.

Nasr al-Din Amir, the Houthi deputy minister of information, said the militants still held the advantage. “We are the ones who are making advances on the ground,” he said. “They are trying to tell the world that they have shifted the balance of power in favor of them, but this a complete and total lie.”

Lt. Gen. Mohammed Ali Al-Maqdashi, Yemen’s defense minister, runs the fight for Marib from a war room burrowed deep into the side of a mountain—an effort to avoid Houthi missile strikes. He expressed no hope that peace talks would bring an end to the war.