International relations, usually shortened to IR, studies how states and other actors deal with one another across borders: why they fight, trade, ally and make rules. Its central puzzle is easy to state. Inside a country there is, at least in principle, a government that makes laws, settles disputes and punishes violence. Between countries there is not.
Anarchy without chaos
Scholars call this condition anarchy. In IR, anarchy means the absence of any world government standing above states; it does not mean disorder or constant fighting. Most borders are peaceful most of the time, and letters, ships and payments cross them every day. What anarchy does mean is that no higher authority guarantees a state's safety or forces it to keep its promises. Much of IR theory is an argument about how much that one fact explains.
Sovereignty and the Westphalian story
The basic unit of the system is the sovereign state. Sovereignty has an internal face, a government's supreme authority over its own territory, and an external face. External sovereignty is the principle that no outside power has the right to rule a state's territory or dictate its internal affairs. It comes with a legal equality among states, so that tiny Tuvalu and huge China each cast one vote in the UN General Assembly.
Textbooks usually trace this system to the Peace of Westphalia. The Peace of Westphalia was the set of treaties signed at Münster and Osnabrück in 1648 that ended the Thirty Years' War, a conflict that killed a large share of the population of the German lands, along with Spain's long war against the Dutch Republic. The standard story says that Westphalia replaced a world of overlapping religious and imperial authority with a world of independent, equal, territorial states.
Many historians now call this a myth, or at least a large simplification. The Holy Roman Empire survived until 1806, and the treaties regulated its internal constitution and religious arrangements in great detail, which is close to the opposite of non-interference. The political scientist Andreas Osiander argued in 2001 that the Westphalian myth reads later ideas of sovereignty back into treaties that mostly confirmed an older, layered order. The principles we now call Westphalian took shape slowly over the following two centuries. Stephen Krasner went further in 1999. Krasner argued that states praise sovereignty in public but break it whenever power or interest makes that convenient.
How the system spread
For most of history, most people lived in empires, not in sovereign states of equal standing. The state system spread outward from Europe, first to the Americas after the independence wars of the late 1700s and early 1800s, then to Asia and Africa through decolonization. The United Nations began with 51 member states in 1945 and has 193 today, with South Sudan the newest, admitted in 2011. For most of humanity, then, the sovereign state is a twentieth-century achievement, and for many colonized peoples sovereignty meant the end of rule by outsiders. That is one reason the principle of self-determination is so closely tied to ideas of human dignity.
Levels of analysis
When a war breaks out or a treaty is signed, where should we look for the cause? In Man, the State, and War (1959), the American scholar Kenneth Waltz sorted explanations into three images. Waltz's three images locate the causes of war in individuals, in the internal makeup of states, and in the structure of the international system. The first image blames human nature or particular leaders. The second blames kinds of states, such as dictatorships or aggressive economies. The third blames anarchy itself. Waltz argued that the first two can help explain why a particular war happened, but that only the third image, the structure of the international system, explains why wars keep recurring at all.
Not all important actors are states. Multinational companies, international organizations, armed groups and charities also shape world politics, and later levels return to them.
Worked example: Somaliland
In 1991, as Somalia collapsed into civil war, the northwestern region of Somaliland declared independence. Since then it has held elections, printed its own currency, kept its own army and police, and been noticeably more peaceful than much of the rest of Somalia. By most practical tests it functions as a state. Yet for more than three decades no other country formally recognized it. Israel became the first to do so in December 2025, a move that Somalia, the African Union and many other governments condemned, and Somaliland still has no seat at the United Nations.
The three images help sort the reasons. At the system level, existing states fear that recognizing one breakaway region could encourage others, and the African Union has long defended the borders inherited from colonial rule. At the state level, Somalia's government insists that the territory is part of Somalia. At the individual level, leaders elsewhere weigh their relations with Mogadishu and with powerful neighbors. The case also shows the two faces of sovereignty pulling apart: Somaliland has internal control without external recognition. Statehood, it turns out, is partly a matter of fact and partly a social status that other states grant or withhold, a point the constructivists of level 4 will build on.