Complex sentence: In May 1959, Group 559 was established to maintain and upgrade the Ho Chi Minh Trail, at this time a six-month mountain trek through Laos.
Split into simpler sentences: In May 1959, Group 559 was established to maintain and upgrade the Ho Chi Minh Trail. At this time, a trek on the Ho Chi Minh trail was a six-month mountain trip trek through Laos.