Co-belligerence is the waging of a war in cooperation against a common enemy with or without a formal treaty of military alliance. Generally, the term is used for cases where no alliance exists. Likewise, allies may not become co-belligerents in a war if a casus foederis invoking the alliance has not arisen. Co-belligerents are defined in the Encyclopaedic Dictionary of International Law as "states engaged in a conflict with a common enemy, whether in alliance with each other or not".