Complex sentence: The Fifteenth Army consisted initially of the highly regarded 33rd Infantry Division and the 55th Infantry Division, which attacked from northern Thailand, which had signed a treaty of friendship with Japan on December 21,1941.
Split into simpler sentences: The Fifteenth Army consisted initially of the highly regarded 33rd Infantry Division and the 55th Infantry Division. The latter attacked from northern Thailand, which had signed a treaty of friendship with Japan on December 21,1941.