The new firebase is part of a creeping US build-up in Iraq since troops first returned to the country with a contingent of 275 advisers, described at the time by the Pentagon as a temporary measure to help get "eyes on the ground".
Now, nearly two years later, the troop count has mushroomed to 4087, not including those on temporary rotations, a number that has not been disclosed.
The troops are moving outside the confines of more established bases to give closer support to the Iraqi Army as it prepares for an assault on Mosul - putting them closer to danger.
Yesterday, a US Navy Seal was killed by direct fire about 5km from the front lines north of Mosul after Isis (Islamic State) fighters penetrated Kurdish peshmerga forces, US officials said. It was the third US combat death in Iraq linked to the fight against Isis.
Arizona Governor Doug Ducey identified the slain Seal as Charlie Keating.
The shift to give closer support to Iraqis comes at a time of political turmoil in Baghdad, which is threatening the legitimacy of Iraqi Prime Minister Haider al-Abadi, the key partner for the United States. Iraqi commanders are concerned that the crisis will complicate and slow progress on the battlefield.
It was inside Firebase Bell, a few kilometres outside Makhmour, a small, mixed Arab and Kurdish town on the edge of Iraq's northern Kurdish region, that Marine Staff Sergeant Louis Cardin was killed on March 20 in a rocket attack, days after the Marines arrived.
The area is prone to attack. Isis fighters sneak out at night to position explosives on the roads and have sent a steady flow of suicide bombers to Iraqi Army and Kurdish positions.
"One managed to infiltrate the base here," said Mahmood, pointing toward the main gate of his base, the headquarters of the Iraqi Army's 15th Division, not far from Firebase Bell. The assault, which included five suicide bombers, also took place shortly after the Marines arrived, he said.
"We eliminated them," he said, adding that none of his own men was killed in the attack.
Before the US troops and their M777 Howitzers moved here, the base came under regular fire.
The attacks have since subsided, but Iraqi troops are still struggling to recapture the Isis-held village of Nasr, 12km from the base, despite launching an offensive shortly after Cardin's death.
After an overnight battle, Iraqi forces withdrew to avoid casualties, Iraqi commanders said.
The village was heavily rigged with explosives. Isis sent car bombs.
"It was a fierce fight," said Mahmood, who, like many of the other soldiers here, was with the Iraqi Army in Mosul when it collapsed so spectacularly two years ago. Since then, he has completed 4 months of training with the US-led coalition.
He deemed the Nasr operation a success because dozens of militants were killed, even if the territory wasn't held.
"My regiment isn't specialised in holding ground," he said. "We liberate and then withdraw."
Mahmood chuckled and shrugged when asked if there were still no US "boots on the ground" in Iraq, as President Barack Obama initially repeatedly pledged.
"They've become more active, and for us, it's had a positive result," he said.
Inching gains have helped secure the base near Makhmour, but the Iraqi forces are heavily dependent on American firepower to move forward.
An operation for Mosul itself appears distant, though. It will involve co-ordinating Sunni tribal fighters, Kurdish forces, Iraqi armed forces and Shia and Christian militias, putting US forces in the midst of a potentially drawn-out and complex battle for the ethnically and religiously mixed region.
- Washington Post-Bloomberg
