WASHINGTON — Neat piles of briefing books and red folders stamped “top secret” blanket the large oak desk that once belonged to General George S. Patton. The national monuments are visible through blast-resistant windows. Steps away, in the custody of a one-star general, are the protocols for ordering the shoot down of a hijacked airliner.

For Ashton B. Carter, his Pentagon office is a far cry from the theoretical jousting of Harvard and MIT’s lecture halls, where he spent much of two decades as one of the nation’s leading national security scholars.