The regional war that no one wanted is here. How wide will it be?

Since the outbreak of war between Israel and Hamas nearly 100 days ago, President Biden and his aides have struggled to contain the war, fearing that a regional escalation could quickly draw in American forces.

Now, with the American-led attack on 16 sites in Yemen early Friday morning, there is no longer a question of whether or not there will be a regional conflict. It has already started. The biggest questions now concern the intensity of the conflict and whether it will be possible to contain it.

This is exactly the outcome that no one wanted, presumably including Iran.

Biden’s decision to unleash airstrikes, after resisting calls to take action against Yemen-based Houthi militants, whose repeated attacks on shipping in the Red Sea were beginning to strain global trade, is a clear change of heart. strategy. After issuing a series of warnings, officials said, Biden felt his hand was forced after a barrage of missile and drone strikes Tuesday were directed at an American merchant ship and Navy ships around it.

“This is already a regional war, no longer limited to Gaza, but already extended to Lebanon, Iraq, Syria and Yemen,” said Hugh Lovatt, Middle East expert for the European Council on Foreign Relations. Washington, he added, wanted to demonstrate that it was ready to deter Iranian provocations, so it visibly placed its aircraft carriers and fighters in a position to respond quickly. But those same positions leave the United States more exposed.

Over the course of 12 weeks, attacks against Israeli, American and Western interests have come from Lebanon, Iraq and Syria, eliciting modest and carefully targeted responses from American and Israeli forces. The United States has also issued warnings to Iran, which the Americans say is acting as a loose coordinator. What is notable about the retaliatory strike in Yemen was its scale: using fighter jets and sea-launched missiles, US and British forces, supported by a small number of other allies, struck large numbers of Houthi missile and drone sites.

“We’re in a low-simmering regional war at the moment and that’s what we’re seeing now,” said Colin P. Clarke, research director at the Soufan Group, a security and intelligence consultancy focused on the Middle East.

Biden is walking a fine line between deterrence and escalation, and his aides admit there is no science to the calculus. Tehran and its allies, including Hezbollah in Lebanon, have been careful to support Hamas, keeping their actions within limits, to prevent a broader American military response that could threaten Tehran’s exercise of power in Lebanon, Iraq and Syria.

But how much control Iran has over its proxies is up for debate, and its leaders may also misinterpret American and Israeli red lines.

The Houthis, a small tribe in Yemen backed by Iran, have been among the most aggressive in pushing the envelope, seeking to block international trade routes across the Red Sea and ignoring American and Western warnings to desist.

Western diplomats have said there has been reluctance to counterattack the Houthis, partly to avoid upsetting a truce in Yemen’s civil war, and partly because of the difficulty of completely eliminating their threat. But the Houthis’ repeated attacks on ships, direct fire on American helicopters and Tuesday’s attack on an American cargo ship have left the United States with what officials said is no real choice.

It is unknown how long it will take for the Houthis to recover and threaten shipping in the Red Sea again, as they have promised.

But deeper American military involvement also helps fuel the perception in the broader world that the United States is acting even more directly on Israel’s behalf, risking further damage to the American and Western position as the death toll rises in Gaza . Israel is now defending its conduct against charges of genocide in an international court.

Iran is using proxies like Hezbollah and the Houthis to distance itself from their actions and maintain its credibility in the region, while attempting to avoid a direct attack, which could put the Islamic Revolution and its nuclear program at risk.

But Iran is also being dragged along by these same proxies.

“Iran is really insisting,” said François Heisbourg, a French military analyst. “It’s another reason why they don’t want a war now: they want their centrifuges to run peacefully.” The Iranians do not have a nuclear weapon, but they could enrich enough uranium for weapons use in a few weeks, from the current enrichment of 60% to 90%, he said. “They did 95% of the work.”

Israel is also intensifying its attacks against Iran’s proxies, especially in Lebanon and Syria. After the Hamas attack, Hezbollah in Lebanon began a series of attacks from Lebanon, leading Israel to evacuate citizens close to the conflict.

Subsequently, the Israeli air campaign killed 19 Hezbollah members in Syria in three months, more than double the rest of 2023 combined, according to a count by the Reuters news agency. In the same period, more than 130 Hezbollah fighters were killed by Israel in Lebanon.

Amine Hoteit, a retired Lebanese army general and analyst, listed several objectives of Israeli strikes in Syria: maintaining the focus there and pressuring the Syrian government “to cut off the Iranian supply route.”

U.S. troops deployed in Iraq and Syria to prevent a resurgence of ISIS have been attacked by Iran-backed militias 130 times since Oct. 17, according to the Pentagon’s tally on Thursday, for a total of 53 attacks in Iraq and 77 in Syria . The United States has retaliated on fewer than 10 occasions, usually after American casualties.

Each time, the United States has said its response is intended to deter further attacks and aims to send a message to Iran and its proxies, who operate freely in Iraq and Syria. But no American soldiers were killed. The concern, according to U.S. officials, is that sooner or later one of the attacks will kill troops, and then the response would be much more deadly and could spiral out of control.

On Jan. 4, the U.S. military launched a rare retaliatory strike in Baghdad that killed a militia leader blamed for recent attacks on American personnel, a move condemned by the Iraqi government.

While the Iraqi government is now dominated by parties close to Iran, the American presence has been tolerated largely because of fears that without U.S. help the Islamic State could quickly regain ground.

But Iraq’s Foreign Ministry on Friday condemned attacks against the Houthis in Yemen. “We believe that broadening the scope of the targets is not a solution to the problem, but will rather lead to a broadening of the scope of the war,” the statement read.

While the main focus is on Hamas in Gaza and Hezbollah, the Houthi threat to trade has the potential to have the greatest global impact, as around 30% of the world’s container ships pass through the Red Sea. Europe suspended production for a few days or longer due to disruptions in receiving parts as ships sailed around the Red Sea and Suez Canal.

The United States and more than a dozen other countries have created a coalition to protect maritime shipping, Operation Prosperity Guardian. But the Houthis continued to try to attack ships, whether with Israeli connections or not, and Maersk has decided to suspend all shipments to the Red Sea after a December 31 attack on one of their ships. He has warned his clients to expect significant disruptions and analysts expect higher prices to add to global inflation.

In public speeches this week, Iran’s supreme leader Ayatollah Ali Khamenei and Hezbollah leader Sheikh Hassan Nasrallah have reiterated that they do not want an expanded war. But Clarke, the counter-terrorism expert, said Israel could not afford to be complacent given the grave miscalculation made before October 7 that Hamas was also not interested in a war.

The recent killings that struck at the heart of Iran’s ties with Hezbollah and Hamas have unnerved Iranians who described them in chat rooms and on social media as having been “slapped over and over again.”

Brig. According to Iranian media reports, General Sayyed Razi Mousavi, killed at Christmas in Damascus, had for two decades been responsible for supplying missiles, rockets and drones to Hezbollah in Lebanon and allied militia groups in Syria and Iraq. Mr. Khamenei performed the ritual prayer of the deceased over his body during his funeral, an honor reserved for the most revered subordinates.

Saleh al-Arouri, deputy political leader of Hamas, killed in a drone strike in the heart of Hezbollah’s power base in Beirut’s Dahieh district, was the Hamas member closest to Iran and Hezbollah and the person they trusted more with sensitive messages and facilitation of financing and technical know-how from Iran.

Reporting contribution was provided by Alissa J. Rubin in Baghdad e Hwaida Saad in Beirut.