Some 231 years ago, a group of men gathered together and wrote out a treatise explaining why the English colonies in the continent known as America should become a single, antonymous entity. That treatise is now known as our Declaration of Independence.
There's a school of thought that holds that the movement for independence wasn't necessarily a popular movement. Most people were happy to go on farming the new continent and otherwise making a living in this new place. A group of firebrands in Boston got together and got the movement going. The English government contributed by mishandling things and seeing the colonies as simply a source of revenue--the firebrands were able to parlay that into a source of grievance. As tensions escalated, the English government responded by sending more troops and demanding that colonists vacate their houses in order to allow troops to barrack in them. Eventually a snowball fight escalated and an escaped slave named Crispus Attucks became the first "American" to die in what became the Revolutionary War. The British response to the snowball fight became known as the "Boston Massacre" and claimed several other lives as well. It's fitting that it began with a snowball fight, because it snowballed after that. The already inflamed colonists, who had begun preparing militia in a desire for autonomy and had already done things like throwing tea in harbors to protest taxation from a government they increasingly felt disconnected from, responded by bonding together and marching as an army against their own country. The first British troops died at Concord Bridge.
Eventually, we won a war of attrition. The colonists fought against the most professional army in the world, but they adopted the tactics of hiding, ambushing and even sniping--things that were anathema to a professional European army. The colonists had nearly nothing in terms of pay or logistics and they endured a hellish winter-over, but they persevered. Some because of ideology, maybe some because they didn't have anything else to do. In the end, the English got tired of fighting for some place most English people had never even seen, much less given a rat's a$$ about.
With their new autonomy from England, the colonies banded together. Thirteen at first. Those thirteen newly united colonies became the United States of America.
Happy birthday to the greatest nation on the face of the earth.