The theory of intervention is based on the concept of state sovereignty, which was incorporated into the 1648 Treaty of Westphalia that ended the Thirty Years' War to prevent rulers from meddling in the affairs of their neighbors at the risk of provoking wars. Sovereignty of a state means that it is entitled to exercise absolute control over its territory and people and can choose to govern its realm as it chooses. In a world of sovereign states, where no overarching governmental unit exists, nobody has the right to interfere with the state's activities unless the interference has been explicitly authorized by the state. In other words, by virtue of its sovereignty, every state, no matter how small and weak or large and powerful, has the legal right to be free from intervention by other states.
The problem with this theory is that it is based on an unrealistic assumption: namely, that all states are completely independent actors who are not affected by what other states do and whose activities do not affect other states. Nonintervention is an impossibility for political units sharing the same world, particularly when they exist in close physical proximity. National borders, which demarcate the state's sphere of control, have become increasingly porous, especially in the modern age of surveillance satellites and the Internet. The fact that international law acknowledges that states have the right of self-defense to protect their sovereignty has always been an implicit acknowledgment that intrusions on state sovereignty should be expected.