Complex sentence: Against such heavy odds, General Vang Pao insisted on recapturing Phou Pha Thi, which the U.S. Embassy believed was unnecessary, but by 20 October 1968, the Hmong Clandestine Army had lost about half their strength due to heavy casualties.
Split into simpler sentences: Against such heavy odds, General Vang Pao insisted on recapturing Phou Pha Thi, which the U.S. Embassy believed was unnecessary. However, by that stage the Hmong Clandestine Army had lost about half their strength due to heavy casualties.