Complex sentence: Weaver was then approached by ATF agents and told that they had evidence of his possession and sale of illegal weapons, and offering to drop the charges in return for his co-operation in infiltrating the Aryan Nations.
Split into simpler sentences: Weaver was then approached by ATF agents and told that they had evidence of his possession and sale of illegal weapons. The agents offered to drop the charges in return for Weaver's co-operation in infiltrating the Aryan Nations.