can_join_war_liege_vassal_check_trigger

can_join_war_liege_vassal_check_trigger = {
	custom_description = {
		text = "basic_ally_can_join_because_liege_or_vassals"
		subject = $JOINER$

		# actor is defender, attackers do not include recipient's liege or vassals
		trigger_if = {
			limit = {
				$WARRIOR$ = { is_defender_in_war = scope:target }
			}
			$JOINER$ = {
				trigger_if = {
					limit = {
						# Diarchs can join against other vassals.
						NOT = { this ?= $WARRIOR$.diarch }
					}
					NOR = {
						any_liege_or_above = {
							is_attacker_in_war = scope:target
						}
						any_vassal_or_below = {
							is_attacker_in_war = scope:target
						}
					}
				}
			}
		}

		# actor is attacker, defenders do not include recipient's liege or vassals
		trigger_else_if = {
			limit = {
				$WARRIOR$ = { is_attacker_in_war = scope:target }
			}
			$JOINER$ = {
				NOR = {
					any_liege_or_above = {
						is_defender_in_war = scope:target
					}
					any_vassal_or_below = {
						is_defender_in_war = scope:target
					}
				}
			}
		}

		# it's fine, whatever
		trigger_else = {
			always = yes
		}
	}
}

As written in game/common/scripted_triggers/00_war_and_peace_triggers.txt.