For most of world history, the main political units were empires. consisting of many different peoples (nations) ruled by a central state through local authorities. The system of nation-states that we see today is a relatively recent invention. Nation-states began to emerge in the Middle Ages in Europe, but it took many centuries to solidify their power and boundaries and to develop a common history and culture. We think of countries like France, Britain and Japan as unified, homogenous countries, but in fact they are relatively recent constructs layered over very different regions, cultures and even languages.
The modern system of autonomous, territorial states formally emerged after the 1648 Treaty of Westphalia. The Treaty marked the end of the horrendous Thirty Years War, which killed millions in Europe and was marked by numerous atrocities. The war was originally about religion (Protestant vs. Catholic), but also involved internal conflicts within the power structure of Europe. Exhausted by the slaughter, the warring parties decided to let each local territory go its own way and decide its own religion. The expanded concept of sovereignty that has since emerged means that each nation-state controls its own territory, domestic policies and foreign policies. In other words, there is no legal authority above the state. (This is another way of saying that there is no overall authority, as we said in Unit 1.)
Sovereignty is the bedrock of world politics. For instance, Pakistan strongly objected when the U.S. violated their sovereignty by sending Navy Seals into their territory without permission to kill Osama Bin Laden. The UN Charter acknowledges sovereignty as a primary principle and so does everyone else. That is why it has been difficult to do anything about the war in Syria. In spite of international pressure, Syria insists on maintaining its sovereignty and saying that foreign troops cannot intervene without its permission.
In such a world, every country pursues its own national interest. Nation-states can enter into alliances, coalitions and treaties to advance national interests, (e.g. Nixon and Kissinger signed arms treaties with our then-arch-enemy the USSR/Russia), and work with international organizations such as the United Nations. On the other hand, nation-states may build up their military for defense or aggression.