Complex sentence: For most of the 1941--89 period, training was largely provided by US advisers, though this assistance has not prevented the same generally low levels of effectiveness common to most of the armed forces in the developing world.
Split into simpler sentences: For most of the 1941--89 period, training was largely provided by U.S. advisors. This assistance has not prevented the same generally low levels of effectiveness common to most of the armed forces in the developing world.