Complex sentence: The M56 saw service with U.S. forces in the Vietnam War, where its function (air portable, self-propelled, anti-tank vehicle) was replaced by the troublesome but effective M551 Sheridan which had a fully armoured turret.
Split into simpler sentences: The M56 saw service with U.S. forces in the Vietnam War. Its function as an air portable, self-propelled, anti-tank vehicle was eventually replaced in Vietnam by the troublesome but effective M551 Sheridan which had a fully armoured turret.