Co-belligerence is the waging of a war in cooperation against a common enemy with or without a formal treaty of military alliance. Generally, the term is used for cases where no alliance exists. Likewise, allies may not become co-belligerents in a war if a casus foederis invoking the alliance has not arisen. Co-belligerents are defined in the Encyclopaedic Dictionary of International Law as "states engaged in a conflict with a common enemy, whether in alliance with each other or not".
Historical examples in World War II
Germany and the Soviet Union as co-belligerents in Poland
After the invasion of Poland in September 1939, Nazi Germany and the Soviet Union partitioned Poland according to the terms of the Molotov–Ribbentrop Pact. Although both countries invaded Poland they had no formal alliance; the Pact primarily served as an agreement of mutual neutrality.
Finland as co-belligerent with Germany in the Continuation War
Co-belligerence (Finnish: kanssasotija, Swedish: medkrigförande) is also the term used by Finland for its military co-operation with Germany during World War II. During the Continuation War (1941–1944), 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 Allies 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 (Fascist) Italy and (Militarist) Japan, as well as a number of countries including neutral (Falangist) Spain, belonged to Hitler's Anti-Comintern Pact.
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:
- 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 to securing Finland's fundamental trade routes for food and fuel.
- Germany was allowed to recruit a Finnish Volunteer Battalion of the Waffen-SS which served under direct German command in operations away from Finnish-Soviet border. (It also recruited from non-belligerents Sweden and Spain. Germany did not recruit from countries formally allied with it until 1943 when Italy surrendered)
- The initial Finnish offensive was co-ordinated with Operation Barbarossa (see Continuation War for details of the pre-offensive staff talks).
- Finnish invasion of the Karelian Isthmus (northern part was Finnish territory until 1940) and to a lesser extent the occupation of over a half of Soviet 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.
- 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 invasion far exceeded the territory of pre Winter War Finland. Finland occupied as far as Lake Onega and Finnish troops even crossed the river Svir for a possible link-up with German troops.
- Britain declared war on Finland on 6 December 1941.
- 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.
- Finland also extradited eight Jews (on orders from the then head of the State Police Arno Anthoni, who was deeply antisemite – the Prime Minister of Finland, Paavo Lipponen issued an official apology for deportations in 2000), 76 political prisoners with non-Finnish citizenship and 2,600–2,800 prisoners of war to Germany in exchange for 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.
- Jews were not discriminated against. A number of them served in the Finnish Army (204 during the Winter War, and about 300 during the Continuation War). When Himmler tried to persuade Finnish leaders to deport the Jews to Nazi concentration camps, the Commander-in-chief of Finland Gustaf Mannerheim is said to have replied: "While Jews serve in my army I will not allow their deportation". Yad Vashem records that 22 Finnish Jews died in the Holocaust, all fighting for the Finnish armed forces. Two Jewish officers of the Finnish army and one Jewish member of the Lotta Svärd women's paramilitary organisation were awarded the German Iron Cross, but they refused to accept them.
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 allies and associates of Germany (Italy from 1943, Bulgaria, Romania and Finland from 1944), after they joined the Allies war against Germany.
- Italian Co-Belligerent Army – fighting with the Allies
- Italian Co-Belligerent Air Force – fighting with the Allies
- Italian Co-Belligerent Navy – fighting with the Allies
- 1st Bulgarian Army – fighting with the Soviet Union during the Vienna Offensive
- 1st Romanian Army – fighting with the Soviet Union and the 4th Romanian Army during the Prague Offensive
- "Co-belligerent(s)", in John P. Grant and J. Craig Barker, eds., Encyclopaedic Dictionary of International Law, 3 ed. (Oxford University Press, 2009).
- Mauno Jokipii, Hitlerin Saksa ja sen vapaaehtoisliikkeet, Suomalaisen Kirjallisuuden Seura, 2002, ISBN 951-746-335-9
- Reime, Hannu (8 October 2010). "Un-Finnish Business". Haaretz. Retrieved 30 August 2017.