Three US service personnel were killed and 34 wounded on Sunday after a drone hit a residential quarters at a military outpost in Jordan known as Tower 22, which lies on the border between Iraq and Syria. It is the first time US soldiers have been killed by hostile fire since the start of the Israel-Hamas war, after an estimated 150 attacks by Iranian-based militias on American bases in Iraq and Syria since 7 October. A military response is expected from the US. Joe Biden, the US president, blamed Iran-backed militants for the attack and said on Sunday: “We will hold all those responsible to account at a time and in a manner [of] our choosing.” Iran has denied any involvement in the attacks, but Islamic Resistance in Iraq have claimed responsibility as part of efforts, galvanised by the Israel-Hamas war, to try to drive US troops out of Iraq and Syria. Who are Islamic Resistance in Iraq? Responsibility for the attack on the US base was claimed by Islamic Resistance in Iraq, a term used to describe a loose coalition of Iranian-backed militias that oppose US support for Israel in the war in Gaza. Membership of the group is deliberately vague, allowing each armed group a level of plausible deniability, according to the Atlantic Council. There is evidence suggesting that Iran’s Revolutionary Guards play a coordinating role, the Washington Institute for Near East Policy said. On Sunday, Islamic Resistance in Iraq said they targeted US personnel with drones at three locations in Syria, including two bases near where Iraq, Syria and Jordan meet, but it was not immediately clear whether the group was referring specifically to the attack that killed the US troops. What is Tower 22? Tower 22 occupies a strategically important location in Jordan, at the most north-eastern point where the country’s borders meet Syria and Iraq, serving as a logistics hub for US military units in Syria. About 350 troops are based at a facility that may have been less well defended than bases in Iraq and Syria because it was thought to be in a safe country. Tower 22 is near al-Tanf garrison, which houses a small number of US troops across the border in Syria. Tanf had been key in the fight against Islamic State (IS) and has assumed a role as part of a US strategy to contain Iran’s military buildup in eastern Syria, while also being close to western Iraq. There are an estimated 4,000 US troops in Jordan. The kingdom is one of the few regional allies that hold extensive exercises with US troops throughout the year. What are the implications of the attack? It was always likely that US personnel would eventually be killed by one of the repeated attacks from Iran-backed militias in Iraq and Syria – or from Yemen’s Houthis firing at warships in the southern Red Sea and Gulf of Aden. A drone carrying explosives landed in the upper floors of a US barracks in Iraq in October, but failed to explode, one of more than 150 such attacks. At Tower 22, casualties were caused because the drone struck living quarters, raising questions as to how it was able to evade the base’s air defences. Much will depend on the level of US military response, and whether it will be deemed to act as a deterrent. But it is likely that attacks on US bases will continue, given that drones and missiles are easy to make, and that the attack on Tower 22 was effective in causing casualties at a relatively low cost. The key question is whether the US will strike directly at Iranian targets or possibly inside Iran itself, or focus on the militias in Iraq and Syria. Even the latter option is not without complications; a week ago, Iraq’s prime minister, Shia al-Sudani, complained via a spokesperson that US retaliatory strikes against the Kataib Hezbollah group “blatantly violated Iraq’s sovereignty”. There are about 2,500 US troops in Iraq – although there has been speculation that the Iraqi government will ask them to leave – and a further 900 in Syria, largely focused on fighting the remnants of Islamic State. It is also unclear whether Iran would escalate if attacked, or if the US attack was sufficiently intense. An option could be to threaten oil tankers and other shipping in the strait of Hormuz, replicating the tactics adopted by the Houthis. But the US has said it does not want escalation. “We are not looking for a war with Iran,” said John Kirby, the White House national security spokesperson. “What we want is a stable, secure, prosperous Middle East, and we want these attacks to stop.”