"The type of wars being fought since the end of World War II has changed dramatically from those fought in the first half of the 20th century and before. Wars fought between countries have dropped in number to nearly zero, while the number of wars fought inside individual countries has risen dramatically (see Figure 1). The reasons for the tectonic shift in human conflict from interstate wars to intrastate wars are diverse and interconnected, and they have been discussed at length elsewhere. In short, global alliances such as the North Atlantic Treaty Organization (NATO), the strengthening and maturing of multinational organizations such as the European Union and the G-20, the globalization of corporate interests, and the resolution of most international boundary issues since the end of World War II are among the causes for the decline in conflict between countries. Meanwhile, resurgent ethnic nationalism and the growth of corresponding separatist movements, the global rise of middle classes (the precursor of revolution), and the historically cyclical reincarnation of radical Islam are among the reasons for the increase in internal conflicts."
Strategic Studies Institute: http://www.strategicstudiesinstitute.army.mil/