combined operation; joint operation
Refers to a coordinated military or strategic action carried out by multiple forces or groups together.
The two armies launched a combined operation.
共同作戦 also means a joint operation, but 連合作戦 often implies a larger-scale alliance or coalition, while 共同作戦 can be used for smaller-scale cooperation.
Compound of 連合 (れんごう, 'alliance, union') and 作戦 (さくせん, 'operation, tactics').