The problem with political revolutions is that, with a few exceptions, they don't work. The history of Revolution contains a few success stories, and dozens of failures.

North America

The United States of America is a very lucky place. The first democratic revolution in centuries occurred there, and it worked. A war was fought, and then power passed in an orderly fashion from the King to the new USA. There were relatively few reprisals, Thomas Hutchinson aside. It seems to have been remarkably simple.

Only when we look in more detail do we see how near a thing this was. Leaving aside the considerable probability of losing the war with Britain, the USA was nearly killed several times in its infancy. George Washington's troops, angry at Congress for not paying their salaries, suggested to Washington that he make himself a dictator. Had he not refused (and threatened to execute the officers making the suggestion for treason if they ever brought it up again), American democracy would have ended there and then. Later, Aaron Burr attempted to carve out his own kingdom from the western part of the US. Again, had his communications not been intercepted, an early fracturing might easily have kept the new-formed America weak and easily reconquerable. The Civil War was a final danger; the two major cultural groups which had united to defeat King George III had never really gotten along, and the War between the States was fought to determine whether or not they would remain cohesive.

Everyplace Else

The dangers that America narrowly avoided have been the ruin of most democratic revolutions since. The French Revolution degenerated into mass hysteria, mob rule, and dictatorship. The Russian Revolution, likewise, was quickly subverted by Lenin. China had Mao. Simon Bolivar's assorted South American revolutions all foundered on the fact that South Americans, unacquainted with self-government, had no idea what to do with freedom when they got it (mostly they elected dictators).

The Roundhead British Revolution under Oliver Cromwell set him up as Lord Protector. He then attempted to pass the office to his son, an act with clear suggestions of hereditary monarchy. It was not until the Glorious Revolution of 1688 that a "revolution" (and a typically calm, restrained, English one at that) would in Britain would bring actually democratic improvement.

Why?

What happened in America that didn't happen in all those other places? At this point we don't know, but we can identify several factors.

First, Americans had a tradition of self-government. This meant that when they were granted independence, they knew how to govern themselves and did it with only minor fuss. Compare to the abortive democracies of Bolivar.

America was an ocean away from its ruler. This meant that most of the oppressors were simply unavailable to purge. Contrast with France, where the aristocrats were right there in Paris to kill. America failed to turn into a bloodbath because there was hardly anyone around worth killing.

America had a low population density and almost no landless poor (proletariat). This fact made it difficult to form a proper mob in America; not enough people hanging around the cities with nothing to do. The occasional mob did form (the Whiskey Rebellion), for example, America by and large was ruled by its politicians.

Finally, we come to luck. America was lucky. We got Washington, and Russia got Lenin. Nothing except Washington's own honor could have stopped him from seizing power. Perhaps something about American society produced Washington, something that could be repeated, but we don't know what it was.

Other Ways to Get Democracy

Most of today's successful democracies came about not through bloody revolutions, but through one of two other agencies: bloodless revolution or externally enforced democracy.

Bloodless Revolution

The Indian revolution under Mahatma Gandhi is a key example of a bloodless revolution. It was not precisely bloodless, but all the blood came from the rebels. A successful campaign of passive resistance, conceived, led, and symbolized by Gandhi, pressed the British into withdrawing. Of course, this would have failed under bolder, more dictatorial oppressors, like Nazi's or Stalinists, but it proved sufficient to establish Indian independence. In India, there was no precedent of violent overthrow to serve as an excuse for dictators. The British pulled out voluntarily, leaving no opportunity for mob rule or execution of political enemies.

The Glorious Revolution was also a bloodless one. The change made was so apparently small as to scarcely deserve the name of revolution, but it was crucial: one king (William of Orange)was substituted for another (James II) at the demand of Parliament. This meant that the (democratic) Parliament had power over kings. After this limitation of the king, Britain became less and less a monarchy and more and more a democracy. In Britain, there was no single establishment of democracy; society grew more and more democratic over the years.

Externally Enforced Democracy

In the past few centuries, conquest has begun to become unfashionable. When a nation loses a war, it is no longer absorbed into the conquering nation. Especially after WWI and WWII, nations had their governments overthrown, but remained independent. Democracies sprung up among these nations because the traditional rulers have been removed but not replaced. Sometimes it works, as in the case of modern Italy. Sometimes it doesn't, as in the case of Weimar Germany.