A casus belli is an act or an event that either provokes or is used to justify a war. A casus belli involves direct offenses or threats against the nation declaring the war, whereas a casus foederis involves offenses or threats against its ally—usually one bound by a mutual defense pact. Either may be considered an act of war. A declaration of war usually contains a description of the casus belli that has led the party in question to declare war on another party.