A lot of international law is active but invisible, e.g. in communication, trade and travel. You dial a code to call a particular country. You pay a duty to the customs office when you bring in foreign goods, part of regulations so complicated that there are specialists who do nothing but help companies navigate the rules. You need a passport and visa when you travel (unless you are traveling between countries that have made an agreement not to require visas). However, if you are a sailor on a ship visiting a foreign country, you do not need a passport or visa, because there is an entirely different set of rules. All of these are based on international law.
The paradox of international law is that it is weak but effective. When the Dutch and the British began enforcing freedom of the seas, if a country broke the rule by seizing ships in international waters, their own ships would also be seized. Very inconvenient. So, it was in each country’s self-interest to cooperate and follow the rules. Reciprocity, or the expectation that following the rules will encourage others to follow the rules, is the heart of international law. Likewise, everyone stays in their lane on the freeway - it’s safer.
If the United States is having a trade dispute with the Republic of Dakine, it could throw the Dakinian ambassador into jail. However, Dakine would probably take umbrage and also throw the U.S. ambassador into jail. Things could quickly get out of hand, to the point where the two sides would have no diplomats to communicate with each other. Therefore, things work much better for all sides if everyone follows the law, in this case the centuries-old rule of diplomatic immunity. Under these rules, embassies are considered to be part of the home country, diplomatic pouches are passed through customs unopened, ambassadors ignore their parking tickets, and diplomats caught spying are merely expelled. (In 1979, Iran violated these rules by allowing ‘students’ to take U.S. Embassy officials in Teheran hostage for over a year.)
Another example of following international law: after ten years of extremely complicated negotiations, the United Nations approved the 1983 Law of the Sea Treaty (UNCLOS), which dealt with a variety of subjects such as passage through international straits and 200-mile Exclusive Economic Zones (EEZs) off the coast of each country. However, because of disputes over possible undersea mining, the U.S. never ratified the treaty. (Technology has now been developed for deep sea mining, but American companies cannot participate because the U.S. has not ratified the treaty.)
Nevertheless, the U.S. has claimed the 200-mil EEZ off its coasts, and has respected other countries’ EEZs. It follows the rules because the predictability and stability resulting from everyone following the rules is in their overall self-interest. Reciprocity again.
So, countries obey international law because it is in their own interests that other countries also follow the law. International law thus depends on reciprocity and states’ preference for the order, stability and predictability that occurs as a result of following international norms. The result is that most countries follow international law most of the time.
Collective action helps enforce those norms. Sometimes there is no collective action, as in the League of Nations’ paralysis in the face of Italian and German aggression in the 1930s. However, sometimes it can be effective. For instance, Libya had broken many international laws over the years, including sponsoring assassination attempts, supporting rebel groups and bombing a civilian airliner over Lockerbie, Scotland in 1988. The rest of the world responded by putting economic and travel sanctions on Libya. Libya finally got tired of the problems this caused and started following international laws, giving up their nuclear program and turning over those allegedly responsible for the airplane bombing.
When Saddam Hussein invaded Kuwait in 1990, nobody wanted to tolerate the precedent of his violating international borders – it could lead to serious chaos on disputed borders in many parts of the world. The UN passed resolutions against the invasion and authorizing the use of force, and a 28-member coalition led by the United States pushed him out.
In contrast, the 2003 U.S. invasion of Iraq was not authorized by a UN authorization for the use of force. Without a UN resolution, the invasion violated international law. In another example, China has taken over and built up islands in the international waters of the South China Sea. Russia took over Crimea and has occupied part of Ukraine. Israel has built settlements in the occupied West Bank. Other powerful countries have also broken international laws, especially when it involves national security.