Complex sentence: Between 1961 and 1965 insurgents carried out 17 political assassinations, although they avoided full scale guerrilla warfare until the summer of 1965 when militants began engaging Thai security forces.
Split into simpler sentences: Between 1961 and 1965, insurgents carried out 17 political assassinations. They avoided full scale guerrilla warfare until the summer of 1965, when militants began engaging Thai security forces.