As American pilots fly new airstrikes over northern Iraq Friday, they'll see some very familiar weaponry in the hands of Islamic State forces: Humvees, MRAP transports, American-made heavy machine guns and American artillery.
Islamic State (which also goes by ISIS or ISIL) forces captured the haul of American weapons as the U.S.-supplied Iraqi Army retreated in the face of the extremist onslaught, leaving expensive American equipment littered on the battlefield.
All that raises the prospect that, at some point during these airstrikes, American taxpayer-financed fighter jets will fire on and destroy American taxpayer-financed weapons on the ground.
"It's a hell of an irony, isn't it?" said retired Army Col. Jack Jacobs, a Medal of Honor recipient. "We're running across weapons that we gave these guys."
He said the cache of American weapons does pose a threat to Americans, but that pilots striking from a distance will likely be able to deal with the ISIS ground forces without sustaining casualties.
Read More US conducts another round of airstrikes in Iraq
Jacobs also noted this is not the first time this has happened to the United States. "We saw the same thing in Afghanistan back when we armed the mujahedeen against the Russians," he said. "And then we had to go fight the same guys using the equipment we gave them."
Nathan Freier is an associate professor at the US Army War College who was deployed in Iraq as a lieutenant colonel at the height of the Bush era "surge." He said the ISIS forces are clearly benefiting from the American weapons they've captured. "They are assembling more capability at the hands of a collapsing Iraqi security structure," he said. "To the extent that they can bring a tank or heavy machine guns to bear, that's going to be a problem for the people they run up against."
But Freier cautioned that the weaponry doesn't automatically make ISIS the dominant force on the battlefield. "They're not going to suddenly assemble an armored division," he said. "Their competence in deploying that equipment in a sophisticated manner is highly questionable."
Read MoreObama authorizes targeted airstrikes in Iraq
The specter of U.S. equipment being turned against U.S. interests can be deeply frustrating to military experts. "We're going to be bombing our own equipment," said retired Gen. Barry McCaffrey.
Referring to a U.S. airstrike announced against ISIL guns Friday morning, he said, "That artillery piece was likely have been supplied by the U.S. to the Iraqi Army, although it could have been old Soviet equipment, too."
"Somewhere between two and four of the Iraqi Army divisions—out of 14—took off," McCaffrey said. "Their general went back to Baghdad, and the soldiers all took off."
Asked how he feels about the U.S. military being forced to destroy U.S. equipment, Jacobs recalled his own experiences in Vietnam—and returning there years later to meet with his former enemies.
"You've got to be a realist when you're dealing with deploying military power," he said. "You can't get emotional about it. If you get emotional, you're going to lose."
—By CNBC's Eamon Javers
Read MoreStock futures mildly higher after US strike in Iraq