Complex sentence: The United States Intelligence Community had dealt with terrorism long before the September 11,2001 attacks, including support of guerrillas against the Soviets in Southeast Asia, and other places where the guerrillas' methods may have included terror.
Split into simpler sentences: The United States Intelligence Community had dealt with terrorism long before the September 11,2001 attacks. The said aspects included support of guerrillas against the Soviets in Southeast Asia, and other places where the guerrillas' methods may have included terror.