The Pentagon has halted shipments of some air defense missiles and other precision munitions to Ukraine due to worries that U.S. weapons stockpiles have fallen too low. The decision was driven by the Pentagon’s policy chief, Elbridge Colby, and was made after a review of Pentagon munitions stockpiles, leading to concerns that the total number of artillery rounds, air defense missiles and precision munitions was sinking, according to three people familiar with the issue.