WASHINGTON — The Central Intelligence Agency has run guns to insurgencies across the world during its 67-year history — from Angola to Nicaragua to Cuba. The continuing C.I.A. effort to train Syrian rebels is just the latest example of an American president becoming enticed by the prospect of using the spy agency to covertly arm and train rebel groups.

An internal C.I.A. study has found that it rarely works.

The still-classified review, one of several C.I.A. studies commissioned in 2012 and 2013 in the midst of the Obama administration’s protracted debate about whether to wade into the Syrian civil war, concluded that many past attempts by the agency to arm foreign forces covertly had a minimal impact on the long-term outcome of a conflict. They were even less effective, the report found, when the militias fought without any direct American support on the ground.

The findings of the study, described in recent weeks by current and former American government officials, were presented in the White House Situation Room and led to deep skepticism among some senior Obama administration officials about the wisdom of arming and training members of a fractured Syrian opposition.

But in April 2013, President Obama authorized the C.I.A. to begin a program to arm the rebels at a base in Jordan, and more recently the administration decided to expand the training mission with a larger parallel Pentagon program in Saudi Arabia to train “vetted” rebels to battle fighters of the Islamic State, with the aim of training approximately 5,000 rebel troops per year.