The research reported here was completed in July 2019, followed by security review by the sponsor and the Office of the Chief of Public Affairs, with final sign-off in March 2021.

The challenge of deterring major conventional aggression is taking on renewed impor-tance in an era of strategic competition. But the nature of that competition, which is primarily playing out below the threshold of major war (at least so far), has created a more immediate and persistent challenge for deterrence: the rise of gray zone aggres-sion, as opposed to conventional interstate aggression. We define gray zone aggression as an integrated campaign to achieve political objectives while remaining below the threshold of outright warfare. Typically, such campaigns involve the gradual applica-tion of instruments of power to achieve incremental progress without triggering a deci-sive military response.1