Washington ? The U.S. has often overestimated the ability of Afghan military and police units to fight on their own, according to an independent report released Monday that calls into question the strategy to win the war and bring troops home.

The investigation is the first objective look at the rating system the military has used for the past five years to judge the effectiveness of Afghan troops. Its findings seem to contradict upbeat assessments recently provided by senior military commanders overseeing the war.

The capability of Afghan forces is considered the single biggest indicator of whether the war is going well and is seen as the linchpin in the U.S. strategy since the war began more than eight years ago.