For the past few months, a single senator — Tommy Tuberville — has blocked hundreds of promotions in the U.S. military. Karoun Demirjian, a congressional correspondent for The Times, explains what’s behind the senator’s blockade, and why military leaders say it’s becoming a threat to national security.

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Persons: — Tommy Tuberville —, Karoun Organizations: U.S ., The Times Locations: U.S