After four years of relative peace, Yemen has spiraled back into full-blown war. In a lightning offensive, the Iran-backed Houthi militia has seized control of the country’s Red Sea coastline in recent weeks. The group is now engaged in fierce clashes with Saudi-backed, Yemeni government forces on multiple fronts, and cross-border attacks in Yemen and Saudi Arabia have escalated.

The war has opened a second front in a Middle East conflict that began when the United States and Israel attacked Iran in February, adding to regional volatility, sending energy prices soaring even farther and darkening the shadow over the global economy.

A year ago, Crown Prince Mohammed bin Salman of Saudi Arabia, the country’s de facto ruler, and President Trump signed a strategic defense agreement. The Saudi leader has repeatedly asked Mr. Trump to intervene militarily against the Houthis, part of the so-called axis of resistance led by Iran, along with Hezbollah in Lebanon and Hamas in Gaza.

But Mr. Trump has wavered. He initially rebuffed the Saudi requests, then, over the weekend, told the Pentagon to prepare airstrikes, only to reverse course and decide to hold off, according to Trump administration officials.

Here is how the war restarted, why the Houthis were able to gain ground so quickly, and what the renewed fighting means for anyone with a stake in the conflict.

How did the war reignite?

Yemen was plunged into civil war in 2014, when the Houthis, Shiite Muslims who describe themselves as an anti-imperialist force battling foreign influence, swept into the capital, Sanaa, and pushed out the internationally recognized government. Soon after, a Saudi-led military coalition, backing the government, began a bombing campaign, plunging the impoverished nation into a devastating conflict.

The two sides agreed to a fragile truce in 2022, but that has unraveled in the wake of the U.S.-Israeli war with Iran.

In July, the Houthis accused Saudi Arabia of bombing Sanaa’s airport to prevent an Iranian plane from landing. The group also balked at longstanding restrictions imposed on Yemeni airspace and declared a naval blockade on Saudi shipping in the Red Sea, obstructing a route that the Saudi government relied on to export oil after Iran stymied shipping through the Strait of Hormuz.

A major turning point in the Houthi advance occurred this month, when Houthi forces captured Mokha, a strategic Red Sea port, and Perim Island, located in the Bab al-Mandab Strait, a narrow waterway that is a crucial route for global trade. Control of the territories put the group in a stronger position to attack passing ships.

Last week, the group sharply escalated its attacks on Saudi Arabia, launching missiles and drones at the capital, Riyadh. The Houthis described the attacks as retaliation against hundreds of Saudi airstrikes and other attacks in Yemen over recent weeks. Saudi officials have neither addressed those allegations, nor acknowledged a Houthi claim to have shot down a Saudi fighter jet.

What do the Houthis and Iran want?

When the truce was reached four years ago, the Houthis said they had won the war and entrenched their rule over northern Yemen. But since then, the economic situation in areas under their control has worsened, spurring civilian resentment and putting political pressure on the group.

The Houthis blame Saudi Arabia’s interventions for the suffering in their territory, analysts say. In recent months, the group has demanded that restrictions on planes and ships entering Houthi territory be lifted, including U.N. inspections. The militia has also insisted that Saudi Arabia pay war reparations and fully withdraw from Yemen — meaning the end of Saudi support for the internationally recognized government.

“The Houthis have no incentive to de-escalate,” said Ibrahim Jalal, a Yemeni researcher and policy adviser. He added that Houthi leaders hope that maintaining pressure on Saudi Arabia will lead to more political concessions and add to their authority.

Iran has also played a key role in the militia’s push, according to Baraa Shiban, a researcher on Yemen and the Gulf at the Royal United Services Institute, a London-based research organization.

The Houthis are an Iranian ally, rather than a complete proxy. The group balances its relations with Iran alongside its own calculations.

But if the Houthis are able to further disrupt the global economy and increase pressure on U.S. gas prices, that would give Iran more leverage over Mr. Trump, Mr. Shiban added. Iran is counting on escalation in Yemen to drag the United States deeper into the regional war and “put more pressure on the Trump administration,” he said.

How did the Houthis advance so quickly?

The Saudi-backed government of Yemen is the main bulwark against the Houthis, and nominally controls a swath of southern and eastern Yemen. But throughout the civil war, the government has become weaker and more fragmented. Today, it is effectively a motley coalition of armed groups, overseen by a “Presidential Leadership Council” of eight men. Many of them distrust one another and have competing political goals, although they share a desire to defeat the Houthis.

They are also beholden to Saudi Arabia, which supports the government financially and militarily, and the kingdom leaves little space for its Yemeni allies to make their own decisions.

By contrast, the Houthis have more authority to act independently of Iran. They still rely on Iran for the most sophisticated drones and ballistic missiles, but they have been able to use artificial intelligence and other tools to make advanced weaponry themselves and developed their own supply chains to do so, analysts say.

“The Saudis are used to treating their Yemeni partners as clients, like hired help. They pay them, but they give them no agency,” said Abdulghani al-Iryani, a senior researcher at the Sanaa Center for Strategic Studies, a research group.

During the past few months, as the Houthis stepped up attacks against Saudi interests, some Yemeni government officials have described an opportunity opening up for a counteroffensive against the militia. The Saudi government tried to unite the factions in Yemen and stepped up military support.

But the first test of those efforts — the battle for Mokha and the Bab al-Mandab Strait — was an abject failure, analysts say. The different forces on the government side struggled to effectively coordinate with one another while the Houthis were unified under a highly centralized leadership.

What will Saudi Arabia do next?

Saudi officials have revealed little about their plans, even as they have repeatedly sought more military support from the United States.

The kingdom has many reasons to be reluctant to return to war. Its previous bombing campaign in Yemen turned into a costly quagmire that contributed to the deaths of hundreds of thousands of Yemenis from violence, hunger and disease. Diplomacy has also failed to definitively end the conflict.

But increasingly, some Saudi officials believe that they have no choice but to confront the militia, analysts say.

“The Saudis have a growing kind of belief that peace with the Houthis is impossible,” said Farea Al-Muslimi, a Yemen specialist at Chatham House, a London-based research group. “They try to buy them, they try to bomb them, they try to fight them, they try to visit them, they try to invite them. None of this has worked.”

Even as some observers hope that the conflict can be resolved quickly, Mr. Al-Muslimi fears the worst.

“There’s a lot of people who are excited for it or think that this will be a quick and fast war, but I have heard that line about almost every war in Yemen,” he said. “This is going to be a bloody, horrible war.”

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