More than two years after the U.S. launched airstrikes against Islamic State targets in the Middle East, the commander overseeing the joint campaign said Wednesday as many as 45,000 ISIS-linked fighters had been killed.

"Although it's no measure of success and its difficult to confirm, we estimate that over the past 11 months we've killed about 25,000 enemy fighters. When you add that to the 20,000 estimated killed prior to our arrival, that's 45,000 enemies taken off the battlefield," Army Lieutenant General Sean MacFarland told reporters at a news briefing. "I only tell you this number to provide a sense to the scale of our support and perhaps explain why enemy resistance is beginning to crumble."

MacFarland said civilians and ISIS administration officials have been forced into front line combat jobs including manning checkpoints, making them a less capable and "diminished" force. "We don't see them operating nearly as effectively as they have in the past, which makes them even easier targets for us so as a result they're attrition has accelerated here of late."

He said the coaltion airstrike destroying hundreds of ISIS vehicles escaping Fallujah last month was further evidence ISIS was not as capable as it used to be. "I don’t they would have made that mistake a year ago," MacFarland said.

The U.S.-led coaltion has launched more than 14,000 airstrikes in the two-year war against ISIS. The first U.S. airstrikes struck ISIS in Iraq on Aug. 8, 2014.

The U.S. military has spent over $8.4 billion fighting ISIS since then.

MacFarland later said the estimates of enemy killed and wounded were "squishy," or difficult to nail down precisely. He added that ISIS fighters could "grab a bunch of people minding their own business off the street, throw them in the back of a pickup truck and drop them off at a checkpoint with some AKs and say, 'defend this checkpoint.'"

The fight has also leveled entire neighborhoods, displaced millions and redrawn the Iraqi map. The U.S.-led coalition estimates ISIS has lost more than 40 percent of the territory it once held in Iraq.

The first coalition strikes were spurred by an ISIS push from Mosul a few weeks after the group's initial rampage across Iraq.

Makhmour base was just one of a number of front-line positions overrun in early August 2014, bringing ISIS fighters within just 19 miles of Irbil, the capital of Iraq's Kurdish region.

After three days of airstrikes, the ISIS advance on Irbil was slowed and Kurdish forces retook the base. Two years later, the fight against ISIS has moved west across the Tigris River into Nineveh province and Makhmour has transitioned from an active front line to a sleepy support position.

"If it weren't for the strikes and the heavy artillery (given to the Iraqi army by the coalition), we would still be up in the mountains," Peshmerga solider Ayoub Khaylani said.

"I will not allow the United States to be dragged into fighting another war in Iraq," President Barack Obama said when he announced the authorization for airstrikes in Iraq in 2014. "American combat troops will not be returning to fight in Iraq."

Last Friday, the Pentagon announced about 400 U.S. soldiers would deploy south of Mosul to Qayarah airbase to aid in the operation to retake Iraq's second-largest city. They were among the 560 additional troops that Obama approved for the Iraq mission last month. The Pentagon said there were about 3,800 U.S. forces in Iraq, not including hundreds on temporary duty and not included in the official count.

As the push to retake Mosul ramps up, the scars from two years of costly victories remain vivid.

Sinjar, the small mostly Yazidi town north of Mosul, was retaken by Kurdish forces nine months ago, but it still lies in ruins. While Sinjar is technically "liberated" the vast majority of its residents still live in tented camps for the displaced scattered throughout Iraq's north.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.