Foreign internal defense (FID) is the participation by civilian and military agencies of a government in any of the action programs taken by another government or other designated organization, to free and protect its society from subversion, lawlessness, insurgency, terrorism, and other threats to their security. The focus of US FID efforts is to support the host nation's (HN's) internal defense and development (IDAD), which can be described as the full range of measures taken by a nation to promote its growth and protect itself from the security threats described above. While historically, the United States provided notable, and largely unconditional, assistance to friendly foreign nations following World War II (Truman Doctrine and the Marshall Plan), FID as conducted today has its genesis in the post-Vietnam era when US policy shifted to emphasize that the United States would assist friendly nations, but would require them to provide the manpower and be ultimately responsible for their own national defense. It is important to frame the US FID effort within the context of the US doctrine it supports and to understand how it fits into the HN IDAD program. US military support to FID should focus on assisting an HN in anticipating, precluding, and countering threats or potential threats and addressing the root causes of instability. Emphasis on internal developmental programs as well as internal defense programs when organizing, planning, and executing military support to US FID activities is essential. Although the FID operation is considered military engagement, security cooperation, and deterrence, FID may include or support operations from across the range of military operations to support the HN's IDAD strategy. Accordingly, US military operations supporting FID provide training, materiel, advice, or assistance to local forces executing an IDAD program, rather than US forces conducting IDAD military missions for the HN.