War is a rare event in world politics, but it is always with us. How can we say this? On the one hand, if we note that the number of territorial states in the global system has ranged from fewer than 30 after the Napoleonic Wars to nearly 200 at the end of the twentieth century, that gives us about 400 nondirectional pairs of states in 1816 and about 18,000 pairs today. And even if we recognize that most wars are between neighbors, and thus reduce the possible pairs at war in a given year to the 40 bordering neighbors in 1816 and the 317 in 1993, the potential is never even approached. There were no wars under way in 81 of the 180 years since the modern interstate system came into being, and seldom more than one in any given year. On the other hand, 75 interstate wars led to a total of more than 30 million battle-connected fatalities among combatants, not to mention tens of millions of additional deaths among civilians in the context of these wars.

Another way to make this point is by looking at the frequency of war involvement by the members of the interstate system.