"Sworn enemies" refers to two or more individuals or groups that have made a formal and serious commitment to oppose and fight against each other. Full definition
My kids even liked it and they are sworn enemies of zucchini! (isachandra.com)
Shooting a gun is easy, creating relationships with sworn enemies is something else entirely, a concept this film plays with in a very imaginative way. (archeravenue.net)
Dialogue between rational actors, even between sworn enemies, is possible; if everyone's rationale is understood. (blog.politics.ox.ac.uk)