allies_progress_towards_friendship_effect

allies_progress_towards_friendship_effect = {
	scope:war = {
		every_war_participant = {
			limit = {
				is_target_in_variable_list = {
					name = joined_as_ally
					target = scope:war
				}
				save_temporary_scope_as = ally_temp
				scope:war = {
					war_contribution = {
						target = scope:ally_temp
						value > 0
					}	
				}
			}
			if = {
				limit = { is_defender_in_war = scope:war }
				scope:defender = { save_scope_as = actor }
			}
			else = {
				scope:attacker = { save_scope_as = actor }
			}
			save_scope_as = ally
			if = { # if war leader is AI, directly increase friendship
				limit = {
					scope:actor = { is_ai = yes } 
				}
				scope:actor = {
					progress_towards_friend_effect = {
						CHARACTER = scope:ally
						OPINION = 0
						REASON = friend_alliance
					}
				}
			}
			else = { # otherwise create potential  friendship
				hidden_effect = { #To nudge friendship
					scope:actor = {
						if = {
							limit = {
								NOR = {
									has_relation_friend = scope:ally
									has_relation_potential_friend = scope:ally
								}
								scope:war = {
									war_contribution = {
										target = scope:ally
										value > 0
									}
								}
							}
							set_relation_potential_friend = scope:ally
						}
					}
				}
			}
			remove_list_variable = {
				name = joined_as_ally
				target = scope:war
			}
		}
	}
}

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