Casus belli is a Latin phrase which literally means “occasion for war.” It is often translated as a “cause for war” or a “case for war,” and it is used to describe an incident which becomes a catalyst for a military conflict. As a general rule, a nation must provide the international community with a casus belli before making war on another nation, and if a nation expects to invoke the assistance of allies, such a cause is critical. The term “casus bellum” is also used in the same way.
In some cases, a casus belli may be an incident which directly provokes a war, such as a preemptive strike or invasion. In this case, the international community often supports the conflict, under the argument that a nation must defend itself when it is threatened. In other cases, the cause may be a pretext or justification for war, in which case the grounds for war may be a bit more murky, and a nation may have trouble convincing the international community that its actions are appropriate.
A classic example of a casus belli was the assassination of Archduke Ferdinand in 1914, which provoked Austria-Hungary into war against Serbia, eventually dragging much of Europe into the war as well and sparking the First World War. Another one was the Japanese bombing of the American port of Pearl Harbor in 1941, which provoked the United States into declaring war on Japan, and therefore on the Axis Powers.
In a related concept, a casus foederis, a nation uses a casus belli as a justification for asking for assistance from allies. A casus foederies requires an existing agreement or treaty between two allies. When the United States was attacked in 2001, for example, the North Atlantic Treaty Organization (NATO) invoked Article 5, which states that an attack on any member is considered an attack on all. Assistance could then be offered, since the United States had a clear casus belli in the form of terrorist attacks which appeared to be linked to Afghanistan.
The case for war is not always clear, and sometimes it is heavily manipulated. The circumstances surrounding the Gulf of Tonkin incident used by the United States as a justification for the Vietnam War, for example, has been heavily questioned in retrospect by historians who have suggested that the situation may have been artificially created to manipulate the American public. In other cases, a nation has had a clear casus belli and it has chosen to attempt to resolve the situation through peace talks and negotiations, an approach which many people think is preferable to shooting first and asking questions later.