Complex sentence: It became a battlefield in the war between the United States and North Vietnam, with the Hmong militias and Thai army forces, on one side, and the North Vietnamese Army with the assistance of the Pathēt Lao, on the other, engaged as auxiliary fighters.
Split into simpler sentences: It became a battlefield in the war between the United States and North Vietnam. The Hmong militias and Thai army forces, on one side, and the North Vietnamese Army with the assistance of the Pathēt Lao, on the other, were engaged as auxiliary fighters.