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Casus Belli is a Latin term that translates to cause of war and refers to a justification or reason a nation-state gives for engaging in armed conflict. This concept has been a element of international relations, diplomacy, and legal frameworks for centuries. It is typically invoked to legitimize military actions, whether to defend against an attack, address perceived threats, or respond to violations of agreements or territorial sovereignty. While the term itself comes from Roman times, the concept it represents has been an integral part of human conflict throughout history.

History

The phrase casus belli has roots in Roman law, where it described a situation that justified war. The Romans believed in certain principles of just war, derived from earlier traditions of warfare among Greek city-states. A war had to be justified by some wrong done to the state, a violation of treaties, or defense against aggression. These legalistic ideas of justifiable war influenced both Western legal theory and broader international norms.

During the medieval period in Europe, the idea of a just war was deeply intertwined with Christian theology, particularly through the writings of thinkers like St. Augustine and Thomas Aquinas. They articulated that wars could be justified under divine law if fought for righteous reasons, including the defense of the faith, protection of the innocent, or response to aggression. Although the terminology of casus belli was not always explicitly used, the underlying notion of needing a legitimate cause for war persisted.

In modern history, especially from the 17th century onwards, the concept of casus belli became more formalized in diplomatic and legal contexts. The Treaty of Westphalia (1648), which ended the Thirty Years’ War, laid the groundwork for the modern system of sovereign states. With sovereignty came the expectation that nations could not engage in warfare arbitrarily; they needed legitimate reasons, often framed as casus belli, to justify military actions.

The development of international law in the 19th and 20th centuries further formalized the concept. The Hague Conventions of 1899 and 1907, as well as the establishment of the League of Nations after World War I, sought to curtail the frequency of war by requiring states to resolve disputes peacefully. In cases where war seemed unavoidable, nations often invoked casus belli to justify their actions, though such claims were often scrutinized by other powers or international bodies.

One of the most infamous examples of a manufactured casus belli occurred during World War II when Nazi Germany claimed that Poland had attacked German forces at the Gleiwitz radio station in 1939. This false-flag operation was used by Adolf Hitler as a pretext for invading Poland, marking the start of the war in Europe. Similarly, the Gulf of Tonkin Incident in 1964, where U.S. ships allegedly came under attack by North Vietnamese forces, served as the casus belli for escalating American involvement in the Vietnam War, despite later evidence suggesting the incident was exaggerated.

Post WWII

In the post-World War II period, with the establishment of the United Nations (UN) and its charter, the legitimacy of military actions has been a major focus. Article 51 of the UN Charter recognizes the inherent right of states to self-defense if an armed attack occurs. This has been interpreted as a justifiable casus belli in various conflicts. However, the rise of international institutions and the concept of collective security has placed increased scrutiny on unilateral military actions.

A modern-day example of a casus belli is the ongoing tension between China and Taiwan. China’s claim over Taiwan is rooted in its interpretation of the One-China Principle, where it views Taiwan as an integral part of its territory despite Taiwan operating as a separate democratic government. Although not a conventional war, this situation has led to continuous threats, military posturing, and diplomatic confrontations.

A potential casus belli could be Taiwan declaring formal independence, which Beijing considers a red line. Such a declaration would likely trigger a significant response from China, possibly leading to military action.

Taiwan's history begins with Austronesian peoples, later joined by Chinese settlers in the 17th century, and briefly colonized by the Dutch. Following Japan's defeat in World War II, Taiwan was ceded back to China, but after the Chinese Civil War in 1949, the Republic of China government retreated to Taiwan, while the People's Republic of China was established on the mainland. Since then, Taiwan has developed as a separate democratic state, but China still claims it as part of its territory, creating ongoing tensions over its sovereignty.