By the end of the Seven Years War, Britain was considered the pre-eminent superpower, with a reputation for having the most powerful navy and the most well-disciplined and organized army in the world. Which is probably why they thought they could easily be the shit out of the rebelling colonists, who had never had a standing military and relied on volunteer militias for their defense. In early 1775, the British marched an army out of Boston, intent on attacking a Patriot arms depot at Concord and ending the revolt in Massachusetts. By the next day the British had been forced to retreat back to Boston and the city was encircled by a force which quickly grew to include Patriots from across New England. When attempts to break the siege resulted in high losses for the British, they settled in for a prolonged siege.
The situation in the Thirteen Colonies quickly spiraled out of control for the British. Emboldened by the situation in Massachusetts, Patriots rose up in every colony and forced out the royal governors and other appointed officials, replacing them with new Patriot elected and appointed assemblies which ran things via an implied threat of violence. Though only a third of the population supported the Patriot cause, another third being neutral and the remainder Loyalist, the Patriots were the most organized, had the guns, and were the most willing to overtly intimidate those who disagreed with them. Though some royal governors tried to fight, several in southern colonies by offering freedom to any slaves who were willing to fight with them, such resistance proved ineffective and by the end of 1775 the Patriot assemblies were firmly in control. Debating what type of governments they should setup for themselves, these assemblies sent representatives to Philadelphia in an attempt to create a unified front against the British. A thrown together affair, this Second Continental Congress became the de facto leaders of united colonies, though their ability to lead were fairly limited. Though they did create a national army, led by George Washington because he was the only guy to show up to the meeting wearing a military uniform.
Things were a real cluster fuck for most of 1775. Though the rebelling colonies were somewhat united in resisting the British, they couldn’t agree what the end result should be. Mere weeks after authorizing an invasion of Quebec, the Continental Congress sent the British a petition for peace, quickly followed by a declaration that the British parliament was a bunch of assholes. As one can probably imagine, this was fairly confusing to the British, who just said fuck it and declared the Patriots and anyone who supported them to be traitors who deserved to be hanged. This greatly weakened the position of more moderate Patriots who wished to see conciliation with Britain, sparking more open dialogue of the possibility of independence, which eventually culminated in the passing of the Declaration of Independence in July of 1776. From that point forward, the Thirteen Colonies saw themselves as thirteen independent states.
While this was taking place, the invasion of Quebec did not go well for the Patriot forces. While the Patriots had hoped Quebec would join them in rebellion, a century of New England protestants declaring all Catholics should burn in hell did not really engender all that much support for the invaders. Though initially successful, winter weather and disease resulted in a fiasco of nightmarish proportions and a retreat back south. As well, while the British evacuated Boston in the spring of 1776, they soon after returned that summer with more British troops and German mercenaries from Europe, seizing control of New York City, Providence, and the entirety of New Jersey. They also convinced the Shawnee, Cherokee, and Iroquois to attack the colonial frontier. In the case of the Shawnee and Cherokee, this was rather an easy sell given colonial settlers had started pouring into what is today Tennessee and Kentucky the moment the British appointed governors lost control. The Iroquois were a bit of a tougher sell. Though most chose to side with the British, others decided to side with the Patriots, resulting in the collapse of the once powerful Iroquois Confederacy. To further confuse things, colonists in lands claimed by both New York and New Hampshire declared themselves an independent nation. Called the Republic of Vermont, this new nation sought to ensure its survival by playing the British and Patriots against each other, openly supporting both sides at one point or another throughout the war.
Though 1776 proved to be a difficult year for the Patriots, they did not just sit idle. Recruiting military officers from France, Germany, and Poland, they better trained their army, which successfully retook the majority of New Jersey by the end of the year. In 1777, the British retaliated via a two-pronged assault. An army moving south from New York City took Philadelphia, forcing the Continental Congress to flee, and a second army moved south from Quebec with the goal of cutting New England off from the rest of the rebelling colonies. However, this attack soon became bogged down in northern New York, eventually resulting in the surrender of the entire invading force. Representing the most decisive Patriot victory to date, it gained the attention of many nations in Europe, nations who would be more than happy to see the British cut back down to size.