Nearly two years ago, an errant spark inside a mill caused an explosion so big it destroyed all the building’s equipment and blew its corrugated fiberglass wall 100 feet.

It also shut down the sole domestic source of an explosive the Department of Defense relies on to produce bullets, mortar shells, artillery rounds and Tomahawk missiles. The factory remains offline.

Military suppliers consolidated at the Cold War’s end, under pressure to reduce defense costs and streamline the nation’s industrial base. Over the past three decades, the number of fixed wing aircraft suppliers in the U.S. has declined from eight to three. During the same period, surface ship producers fell from eight to two, and today, only three American companies supply over 90% of the Pentagon’s missile stockpile.

That’s emerging as a gnawing problem for the U.S., whether in supplying weapons and ammunition to Ukraine or in restocking reserves to prepare for a potential confrontation with China in the new era of great-power competition, according to U.S. military officials, defense experts and congressional staffers.