An attack or other use of force against the US, US forces, or other designated persons or property. It also includes force used directly to preclude or impede the mission and/or duties of US forces, including the recovery of US personnel or vital US Government property. (JP 1-02 & JP 3-28).
Source: Terms & Definitions of Interest for DoD Counterintelligence Professionals Office of the National Counterintelligence https://www.dni.gov/files/NCSC/documents/ci/CI_Glossary.pdf