Jack Teixeira was arrested by the FBI at his family home in Dighton, rural Massachusetts, after being identified as the leader of an online chat group where the documents first emerged.
Dozens of leaked documents revealed US assessments of the war in Ukraine as well as sensitive secrets about American allies.
BBC security correspondent Frank Gardner examines the implications of the leak.
Video by Jack Burgess, Gianluca Avagnina and Ameer Ahmed