There’s no such thing as international “law.” There is no higher authority to enforce a single code of conduct. In the 19th Century, what people called international “law” was really two things.
One was the belief that treaties between countries were binding contracts. In 19th Century diplomacy, bi-lateral treaties (treaties between two countries) were important. Multi-lateral treaties (treaties between a bunch of countries) were more important. What you say that you will do, you must do. Otherwise, the contract is broken. Not only may the other party or parties make other arrangements, but they can beat up on you if they have the means and the mind to do so.
The second was that there were standards of behavior. They weren’t particularly high standards. Even so, people often had trouble meeting them. Just as you were not supposed to cheat at cards in your personal life, you were not supposed to lie to another country. To lie to another country, you had to respond to a direct question in an untruthful manner. However, if you phrased things in such a way that they got the wrong idea all on their own, that wasn’t a lie.
If you were going to try something new, you should talk to the other powers beforehand. Certainly, you should talk to the other countries with what was called an “interest” in the matter. An “interest” meant having a “common concern” with others in something, or a “right to a share” in something. If something that some other country did would have an effect on your country, you had an “interest” and a right to a voice. If your country had signed a treaty, your country had an “interest” in the operation of that treaty. If some country did something that had no effect on your country, then you didn’t have an “interest” and your country had no right to be consulted. If your country had not signed a treaty, your country had no right to a voice on the treaty.
That said, the various meanings of the legal term “interest” can conflict. For example, back in the 1990s, Iraq and Kuwait were at odds over a matter. The ruler of Iraq, Saddam Hussein, told the American ambassador, April Glaspie, about his grievances. Ambassador Glaspie responded that the United States couldn’t offer an opinion on the dispute because it had no “interest” in the matter. It wasn’t a party to the dispute. Then Saddam Hussein sent his army to occupy Kuwait and to threaten Saudi Arabia. Well, that’s where the oil comes from. So, that touched on an American “interest” of a different sort. If Saddam Hussein thought that the United States was going to let one country monopolize the oil supply of the Western world, he was very much mistaken.
You were supposed to try to settle disputes peacefully, without resorting to war. War is an uncertain business. Lots of things can go wrong. Usually they do go wrong. Flunk a war and you can have all sorts of problems. Win a war too decisively and other countries will start to worry about you. So, dodging war in favor of talk served everyone’s interests.
 It is possible to discern two different attitudes at the core of this belief. On the one hand, until recent times, bankruptcy was seen as a form of fraud. You could get sent to prison if you could not pay your debts to people who had loaned you money in good faith. This happened to the father of Charles Dickens. Sir Walter Scott worked himself near to death out of the felt obligation to help pay off the debts of his publisher. On the other hand, diplomats were almost always aristocrats. They came out of a culture of personal honor and—if necessary—dueling.
 The American Secretary of State John Foster Dulles was particularly adept at this.
 Shamefully, Glaspie was made the goat during the panic in Washington that followed. She ended her career as Consul General in Cape Town, South Africa.