Due to increasing Cause_change_of_position_on_a_scale U.S. intelligence of the build-up of regimental-size PAVN units operating Being_in_operation in South Vietnam , the increased Cause_change_of_position_on_a_scale American military presence Presence in that country , and the initiation of Operation Rolling Thunder , American planners in Washington and Saigon decided Deciding that the bombing Attack in southeastern Laos should be stepped up Cause_change_of_strength .