Phony electronic parts have wound up at the U.S. Missile Defense Agency seven times in the past five years, its director told Congress on Tuesday. None of the fakes were actually deployed in active combat situations. But if they had, it might have imposed “a cost that could be measured in lives lost,” Lieutenant General Patrick O’Reilly warned the Senate Armed Services Committee Tuesday.

The flood of counterfeit goods creeping into military systems spotlights how vulnerable defense contractors’ supply chains have become, and how tricky it is to regulate them. Because the military tends to use i...