The drone, packed with explosives, used the civilian flight path into Irbil airport to disguise its intent. It crashed into a CIA warehouse on the American airbase beside the civilian airport in April. US forces described feeling the shockwave across the base, which now has the biggest concentration of US and British forces in Iraq. The warehouse was left in ruins, but no-one was hurt. "They knew what they were hitting, but they didn't know what was inside," said one US commander. For the last year and a half there have been some 300 attacks on US interests in Iraq, mostly rockets, as well as improvised explosive devices targeting supply convoys. "This is a game changer," the commander said. The drone was Iranian made, military grade, and a greater threat because of its precision. Drones are also fiendishly difficult to stop. Iran denies involvement in the attacks. Militia threat Since the US assassinated the top Iranian general Qasem Soleimani and Abu Mahdi al-Muhandis, the leader of the Iranian-backed Iraqi militia Kataib Hezbollah, in a drone strike in Baghdad in January 2020, the shadow war between the US and Iran on Iraqi soil has shifted gear. US forces, currently a brigade from the Louisiana National Guard, are stationed at the base in Irbil, which is in the northern Kurdistan Region. They are part of the international coalition, which includes British troops too, supporting Iraqi and Syrian Kurdish forces in the last stages of the fight against the Islamic State group. But they do not engage IS in combat operations - the biggest threat they face is from Iranian-backed groups. There are only 2,500 US service members on the ground in Iraq and north-eastern Syria now, a figure set by former President Donald Trump, and unchanged by his successor Joe Biden. In a non-binding resolution passed after Soleimani and Muhandis were killed, the Iraqi parliament called on foreign forces to leave. The figure does not account though for special operations forces. Ride a Chinook from Irbil airbase and it will be as crowded with bearded special forces personnel as with regular troops.