How do nations decide when to go to war? What are the rules that govern when it is permissible to resort to war under international law? Consider the reason for the United States–led invasion of Iraq in 2003. Much was made of Iraq’s supposed ability to create weapons of mass destruction (WMDs) that could have endangered the citizens of the United States and its coalition partners. Indeed, the need to defend against WMDs served as the justification for the United States coalition to strike first. This is an example of a nation using self-defense as a rationale for attacking another state.

The legality of this sort of pre-emptive attack is controversial. The arguments around it have their roots in a long history, stretching back at least to the end of the Napoleonic Wars, according to Isabel V. Hull, History.

“The UN Charter says you’re only allowed to go to war in self-defense, or if the Security Council says that a coalition of states can go to war to stop an occurrence, such as genocide, that breaks the rules of the charter,” Hull explains.

Most law and history books identify the establishment of the United Nations in 1946 as the first clear and inarguable moment when states were reduced that far in their ability to go to war. Others may choose the Kellogg-Briand Pact of 1928, which renounced the use of war and called for settling disputes peacefully. Hull, on the other hand, looks to the end of the Napoleonic Wars and the Congress of Vienna in 1814-15.

“What you see in the various crises that come after 1814 to 1815 in Europe is a very clear pattern of recognition among European states that there is a whole series of things for which you can’t use an army and must use diplomacy instead,” she says.

Jus ad Bellum, the Right to War

Hull is currently working on a large research project that will culminate in a book. The book will concentrate on what is called the long nineteenth century—1814 to 1914— and the criteria known as jus ad bellum, which identify when war is permissible under international law.

“If you look at the long nineteenth century, there are no wars in Europe aside from the German and Italian wars of unification,” Hull says. “Why? After the Napoleonic Wars, which went on for 23 years, the European powers at the Congress of Vienna set up a European-wide system to prevent wars of aggression. They set down the state borders and said, ‘This is how things are and this is how they’re going to stay, and you’re not allowed to use war to change what we’ve set down.’”

“It’s worthwhile for people to understand what a very long pedigree these international rules have and how many millions of people have died to put them in place.”

While the Congress of Vienna did not produce written jus ad bellum rules, Hull hypothesizes that European state practice during the nineteenth century created unwritten customary law regarding when war could be waged. “In order to be binding, an international law doesn’t have to be written down,” she says. “Customary law is equally law, and much of international law in the nineteenth century, including the laws of war, were not written down.”

To discover what nineteenth-century Europeans considered customary law on the behavior of states going to war, Hull takes a hard look at international crises of the time. She studies letters between political leaders and their public and private statements about the situation. “I’m looking at the arguments they made about what should happen, what was happening, and what could be done about it,” Hull says. “Those kinds of discussions inside governments and among governments give an idea of what people at the time thought the customary norms and rules were.”

Exceptions to the Rules of Initiating War

Some exceptions to the rules did allow some wars to be waged. These included self-defense, intervention on behalf of the peace of Europe, and national self-determination, which grew into an expansive notion of state sovereignty in Imperial Germany by 1914.

“The latter day German argument was that state sovereignty meant that the state could do whatever it wanted in its own self-interest,” Hull explains. “That idea was behind the outbreak of war in 1914. Both Germany and Austria saw the war as a preventive one, waged to stop France and Russia from becoming so powerful that Germany’s war plan would have been ineffective.”

World War I, an Error?

Contrary to a common belief that World War I was caused by a series of errors or by defensive alliances between major powers, Hull believes Germany’s decision to go to war was carefully thought out and predicated on its own interpretation of the unwritten jus ad bellum rules.  “I think Imperial Germany launched that war on purpose,” she says. “The war doesn’t make sense if you think it was just a traffic wreck. That’s not at all what happened. All the powers knew what was at stake. That’s why it lasted so long. If it had really been an error, presumably statesmen could have gotten out of it. But nobody except Austria wanted to negotiate after 1914, because the stakes were nonnegotiable.”

Hull continues, “If you’d asked, Germans would have said, ‘We can’t make peace because the allies would insist on upholding a state order inimical to our interests to expand and develop. Our population is too big and our economy too large for us to have a reasonable chance of success if they keep holding us in.’ The German point of view was an expansive notion of self-determination. If the central powers had won, Germany would have overthrown the 1814-15 rules of settlement and become the Hegemon of continental Europe. The allies, on the other hand, believed Austria—and especially Germany—were guilty of starting an aggressive war that contravened the customary rules governing state behavior in Europe. Their position is clear in the Treaty of Versailles, which ended World War I. It held Germany and its allies responsible for all the consequences of beginning an aggressive war, including economic reparations.”

Today, with various governments around the world flaunting the rules that govern going to war as well as the rules of engagement, Hull sees her research as especially salient. “It’s worthwhile for people to understand what a very long pedigree these international rules have and how many millions of people have died to put them in place,” she says. “International law is very robust because so many states have an interest in it. It’s not some frail lace you throw over a cannon and then say everything will be good. It’s the vast majority of people understanding that certain forms of state interaction are unacceptable. That’s certainly what the folks learned after Napoleon.”