An upgraded class of high-powered weaponry has flooded the battlefields of the Middle East, threatening even the most sophisticated battle tanks and highlighting a gap in U.S. military preparedness.

The weapons—antitank guided missiles, or ATGMs—were first developed decades ago, but recent years have brought advances in their technology, ease of use and availability on the battlefield, making them a fearsome and little-acknowledged threat to U.S. troops.

The proliferation is largely the result of efforts of powers such as the U.S., Russia and Iran to arm and train proxy fighters, including by sending them antitank missiles along with other weaponry.

A U.S. program begun in mid-2013 provided weapons including ATGM missiles to rebels fighting the Assad regime in Syria. President Trump later canceled the program, saying in a 2017 interview with The Wall Street Journal that it allowed weapons to fall into al Qaeda hands.

“There is absolutely the possibility that the U.S. may face some of the same ATGMs it has delivered in the past to the Middle East,” said Omar Lamrani, a senior military analyst with the Austin, Texas-based defense-intelligence firm Stratfor. Islamic State and al Qaeda offshoots, among others, now possess American-made missiles, he added.