Back in the 1700's, the German philosopher Immanuel Kant wrote a book entitled _Perpetual Peace_, in which he argued that a Europe made up of free republics would be able to guarantee peace. He reasoned that few peoples would elect leaders who promised to invest their sons' lives in wars; whereas in Kant's time, war was the sport of kings.
Indeed, in America, war presidents have generally had an easy time, unless, like Lincoln, their generals racked up key victories in the nick of time or, like Franklin Roosevelt, Americans perceived themselves as recipients of an unprovoked attack and the media liked the president. Further, the democratized Europe that has existed since the end of World War II has been peaceful; while the end of the Communist bloc has further continued a long peace, save for Ulster and the disintegration of Yugoslavia.
Hence many political scientists such as Zeev Maoz, Rudolph Rummel, and others extend Kant's theory to one that posits a "democratic peace". the thesis holds that democracies generally do not fight each other, even if they do go to war with dictators.
As a former US diplomat, I would like to believe that the democratic peace thesis is true. However, part of me remains skeptical. Peace probably depends on a very complex set of political, cultural, economic, demographic, geographic, and other factors.
The democratic peace thesis won a notable convert in George W. Bush, who, in the wake of 9/11, abandoned his distaste for the failed Clintonian nation-building exercises in failed Somalia and Bosnia, and proceeded in a so-far unsuccessful attempt to democratize Iraq and Afghanistan, and support the further democratization of the rest of the Middle East. A similar American mindset continues to manifest itself with Hillary Clinton and Samantha Power pressing Pres. Obama to come out in favor of "democratic" uprisings in Tunisia, Libya, and Egypt--even when it is doubtful that such uprisings are democratic at all.
Seymour Martin Lipset has noted that for most of the 20th century, democracies have faced common enemies, a situation which is likely to place potential conflicts among themselves on the back burner. Yes, the war for the Atlantic between the US and Britain which strategic thinkers posited as "inevitable" between the early 1800's and 1930's never materialized, but perhaps the reason it never did was because the British and US navies were too busy supporting each other as allies in two world wars and the Cold War. Further, Britain and the US fought each other in 1812-15, while Britain fought the Boers of South Africa who, while nasty towards their non-white neighbors, were democratic in running their own affairs. The USA itself suffered a devastating civil war between two sections of an all but universally democratic nation. Even Switzerland had its Sonderbund War in `1848. All these examples suggest that war between democracies are possible.
Of course, a common consensus among the elites of many states in favor of Kant's proposal for international peace may also be at work in the "democratic peace" we have now. If so, long may it flourish. Yet powerful ideological counter-currents may yet overcome this. Conceivably, a democratized public may develop a taste for imperial adventure (as happened in Britain and France in the late 19th century), and which may well appear in other parts of the world. The forms these may take are perhaps unimaginable at present, but may be fully comprehensible should they appear.
Aristotle, in the 4th century B.C., noted that rule by the many is subject to corruption as soon as the majority realizes it has the political power to "constitutionally" loot the minority. The polity sees its chief enemies in its neighbors rather than in external threats, and some factions may even be tempted to align themselves with external enemies for domestic political advantage. This situation may bring social disintegration to a modern nation state as easily as it brought it to certain ancient Hellenic city-states. A disintegrating state falling into out-and-out failure is certain to attract both potential predators and well-meaning interventionists. Some of these may, conceivably, be other democracies.
Yes, I'm all for maintaining the democratic peace, and if it spreads beyond a few favored northern lands, I will applaud. But the democratic peace is not something on which I would bet the proverbial farm.