The first clue that something had changed in the U.S. approach to selling military equipment to Europe came as Denmark neared a decision on the purchase of a multibillion-dollar air-defense system. For weeks, American and French negotiators had aggressively pursued the deal. But as the deadline approached, the Pentagon suddenly lost interest.

“We couldn’t understand why,” a contractor who had been tracking the discussions told me. “It seemed like a no-brainer, but they just weren’t into it.”

The comments surprised some State officials, but they soon learned that it wasn’t just Denmark having its access cut off. Current and former administration officials told me the Pentagon has identified some weapons as being in short supply, and is moving to block new requests for those systems coming in from Europe. It wasn’t immediately clear to those I spoke with how long the hold will last, how many weapons are on the list, or if it could expand to include even more weapons. Few exemptions will be granted.