vassal_join_rebel_war_effect

vassal_join_rebel_war_effect = {
	save_temporary_scope_as = joining_vassal
	scope:liege_temp = { custom_tooltip = tooltip_fellow_vassal_joins_war } #The war has not started when viewing the interaction
	hidden_effect = {
		scope:rebel_temp = {
			every_character_war = {
				limit = {
					using_cb = refused_liege_demand_war
					casus_belli = {
						primary_attacker = scope:rebel_temp
						primary_defender = scope:liege_temp
					}
				}
				add_to_list = war_to_join
			}
		}
		every_in_list = {
			list = war_to_join
			limit = {
				# Bloc members are already pulled in by bloc_member_join_war_effect above
				NOT = { is_participant = scope:joining_vassal }
			}
			hidden_effect = { set_called_to = scope:joining_vassal }
			add_attacker = scope:joining_vassal
		}
	}
	hidden_effect = {
		scope:liege_temp = {
			add_opinion = {
				target = scope:joining_vassal
				modifier = rebellious_vassal_opinion
			}
		}
	}
}

As written in game/common/scripted_effects/00_interaction_effects.txt.