Co-belligerence

Co-belligerence is the waging of a war in cooperation against a common enemy with or without a military alliance. Generally, the term is used for cases where no formal treaty of 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".

Under the Fourth Geneva Convention, nationals of a co-belligerent state are not regarded as protected persons if their state has normal diplomatic relations with an allied nation. Article 4 of the convention states:

[N]ationals of a co-belligerent State, shall not be regarded as protected persons while the State of which they are nationals has normal diplomatic representation in the State in whose hands they are.

In other words, it is not a war crime under international humanitarian law for foreign co-belligerent citizens to be subjected to atrocities whether in their own territory or in occupied territory by allied belligerent troops. Much like in the time of peace, such wartime atrocities would fall under the co-belligerent nation's domestic law or the allied belligerent's own military law. The International Committee of the Red Cross (ICRC) commentary of 1958 stated:

The case of nationals of a co-belligerent State is simpler. They are not considered to be protected persons so long as the State whose nationals they are has normal diplomatic representation in the belligerent State or with the Occupying Power. It is assumed in this provision that the nationals of co-belligerent States, that is to say, of allies, do not need protection under the Convention.

There are certain exceptions to this rule, however. On a judgement issued on July 15, 1999 on The Prosecutor v. Duško Tadić case, the International Criminal Tribunal for the former Yugoslavia (ICTY) appeals chamber noted that nationals of a co-belligerent state would be afforded the status of "protected persons" under the Fourth Geneva Convention if they "are deprived of or do not enjoy diplomatic protection."

After the invasion of Poland in September 1939, Nazi Germany and the Soviet Union partitioned Poland in accordance with the terms of the Molotov–Ribbentrop Pact. Although both countries invaded Poland, they had no formal, open alliance; The pact was formally an agreement of mutual neutrality. German and Soviet cooperation against Poland in 1939 has been described as co-belligerence.

Co-belligerence (Finnish: kanssasotija, Swedish: medkrigförande) was also the term used by the wartime government of Finland for its military co-operation with Germany (who they called their "brothers-in-arms") 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 described as a direct consequence of Germany's attack on the Soviet Union, Operation Barbarossa.