Co-belligerence is a broader and less precise status of wartime partnership than a formal military alliance. Co-belligerents may support each other materially, exchange intelligence and have limited operational coordination. The aims of war in which co-belligerents participate may differ considerably.
The term co-belligerence indicates remoteness between the co-belligerent parties, cultural, religious, ideological or otherwise, whereas alliance indicates a corresponding closeness.
The term was used in 1943–45 during the latter stages of World War II to define the status of former German allies and associates (chiefly Italy, but also from 1944 Romania, Bulgaria, Hungary and Finland), after they joined the Allied war against Germany.
Co-belligerence is also the term used by Finland for her military co-operation with Germany during the Second World War. Between 1941 and 1944, during what the Finns called the Continuation War, both countries had the Soviet Union as a common enemy. Finnish reentry into World War II was a direct consequence of Germany's attack on the Soviet Union, Operation Barbarossa.
While the Allied states often referred to Finland as one of the Axis Powers, Finland was never a signatory to the German-Italian-Japanese Tripartite Pact of September 1940. The Allies, in turn, pointed to the fact that Finland, like Japan and Italy, as well as a number of countries including neutral Spain, belonged to Hitler's Anti-Comintern Pact.