In March 1947, the United States established an economic and military assistance program to bolster the nationalist Greek government against a communist insurgency. The Greek government suffered from a collapsed economy, deep social divisions, and an inability to defeat the insurgents in battle. The Joint US Military Advisory and Planning Group provided operational advice to the Greek National Army that improved the nationalists' aggressiveness, tactics, battlefield management, and logistics. The advisors used training, mentorship, directive control, and disciplinary action to affect the nationalists' combat leadership. The improved leadership led to more effective combat operations against the communists. These operations pressured the insurgency, which had alienated Yugoslavia and committed to fighting with conventional tactics. These two insurgent errors, the massive economic and military aid program, and the improved nationalist combat performance resulted in a decisive victory in August 1949. The study provides insight into how advisors can affect a military's leadership.