The American victory in the Revolutionary War was a huge upset. There were several factors that led to this happening.
The British had poor military leadership. Many of their generals got their posts because of whom they knew, and not because of what they knew. The generals didn’t follow the plans and made poor decisions. General Howe went to Philadelphia instead of Albany. General Burgoyne carried so many luxury items; he had to move very slowly. This gave the colonists an opportunity to regroup when needed. Because General Howe never showed up in Albany, and because another army was defeated, Burgoyne got beat at Saratoga.
Once the British lost this key battle, this opened the door to foreign aid. France and Spain now began to help us by providing us supplies and money. Some professional soldiers also came from Europe to help us.
The Americans had a cause for which to fight. Some Americans were literally fighting for their life. If the colonists would have lost, some colonial leaders may have been killed. Plus, the colonists were fighting for their independence. This was very motivating. The British used hired soldiers who were fighting for pay.
We also knew the land quite well. The British, who had to defend the entire colonial area, didn’t have the same degree of knowledge about the land that the colonists had. This helped the colonists tremendously.
We benefited from excellent military leadership. George Washington was a fantastic general and leader. He may have been our biggest advantage.
Because of these factors, the unexpected happened. The colonists got their freedom from powerful Great Britain.