Co-belligerence

Co-belligerence is the waging of a war in cooperation against a common enemy without a formal treaty of military alliance.

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 Allies as co-belligerents with former enemies
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.

Finland as co-belligerent with Germany in World War II
Co-belligerence is also the term used by Finland for her military co-operation with Germany in the Continuation War of 1941-44, when both countries had the Soviet Union as a common enemy. The Continuation War was a direct consequence of Germany's attack on the Soviet Union, Operation Barbarossa. Until then the German and Soviet governments had been allies, as during the Winter War (1940) against Finland.

While the Allied propaganda from 1941 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.

Remaining sympathy among the western Allies, from the turn of the century Russification of Finland, to the later Finnish Civil War and Finnish cooperation with Franco-British interventions in the Russian Civil War, and again enhanced during the Winter War, may have contributed to an Allied assessment of Finland that, despite the state of war, was more understanding than in the cases of Hungary and Romania.

Adolf Hitler declared Germany to be im Bunde (in league) with the Finns, but Finland's government declared their intention to remain first a non-belligerent country, then co-belligerent after the Soviets started bombing Finnish cities all over the country, not the least due to a remaining neutralist public opinion. The truth was somewhere in-between:
 * 1) By mining the Gulf of Finland Finland's navy together with the Kriegsmarine before the start of Barbarossa locked the Leningrad fleet in, making the Baltic Sea and the Gulf of Bothnia practically domestic German waters, where submarines and navy could be trained without risks in addition of securing Finland's fundamental trade routes for food and fuel.
 * 2) Germany was allowed to recruit a battalion from Finland which served under direct German command in operations away from Finnish-Soviet border. (It also recruited from non-belligerent Sweden and Spain. Germany didn't recruit from countries formally allied with it until 1943, when Italy surrendered)
 * 3) The initial Finnish offensive was co-ordinated with Operation Barbarossa (see Continuation War for details of the pre-offensive staff talks).
 * 4) Finnish re-conquest of the Karelian Isthmus (historically Finnish territory), and to a lesser extent the occupation of East Karelia contributed to the Siege of Leningrad. Finland also helped to block Soviet supply deliveries into the city and hosted, supplied and participated within the Lake Ladoga Flotilla which aimed to disrupt Soviet supply delivery.
 * 5) A German army corps invaded the Soviet Union from Finnish Lapland, and German army and air force units reinforced the Finnish army during the decisive 1944 battles on the Karelian isthmus. Finland and Germany executed several joint German-Finnish Operations at the Finnish front. The Finnish conquests far exceeded the territory of pre Winter War Finland. Finnish conquest ranged as far as Lake Onega and Finnish troops even crossed the River Svir for a possible link-up with German troops.
 * 6) Great Britain declared war on Finland on 6 December 1941.
 * 7) Germany supplied Finland with military equipment of all kinds, ranging from weapons, uniforms and helmets to tanks and assault guns. Finland in exchange delivered rare resources like nickel.
 * 8) Finland also extradited 8 Jews, 76 political prisoners with non-Finnish citizenship and 2,600-2,800 prisoners of war to Germany in exchange of 2,100 Fennic/Karelian prisoners of war from Germany. Some of the extradited had Finnish nationality, but had moved to Soviet Union before the war, received Soviet citizenship and returned to Finland in secret.