European Union
There haven't been any international wars within the borders of the EU, which was one of the main aims of the early union in the first place. The theory was that free trade of steel and coal (vital for waging war) between France and Germany would make a war between them impossible. Some argue that the EU encouraged the development of democracy in Spain, Portugal and Greece.
Democracy
The argument is that democratic countries don't tend to go to war, so the spread of democracy has made war less likely.
Global Capitalism
As multinational corporations spread across borders, a war (supposedly) becomes economically destructive for both countries. Capitalism makes countries interdependent.
Decline in ethnic nationalism
Racism and ethnic-nationalism have become much less acceptable, probably because of the destruction caused by racist nationalism in the world wars.
Success of ethnic nationalism
European wars ended with borders based on ethnic lines - lots of small countries instead of large multiethnic countries. With stable borders and low levels of ethnic diversity within them, the continent was at peace. (Yugoslavia and USSR failed as multiethnic states.)
Pax Americana
American military domination of NATO and political influence makes war between its allies unlikely. Aging populations Low fertility rates cause populations to age, and older people are less inclined towards violence. Economic pressures force young people into the market, out of the army.
Any more? :D
