|"Is this all real? Or has this been happening inside my head?"
The topic of this article is of a real-life subject that has been mentioned "in-universe" in a canon source. The Harry Potter Wiki is written from the perspective that all information presented in canon is true (e.g., Hogwarts really exists), and, as such, details contained in this article may differ from real world facts.
The Revolutionary War was a war fought between American No-Majs and British Muggles. It began when thirteen of the North American states declared themselves as the United States of America and therefore independent from Great Britain's rule. It lasted from 1775 until 1783.
When the war started, the witches and wizards from either country had to decide whether they were to intervene and fight. In 1777, it led to the great 'Country or Kind' debate in Washington, North America. Then President of the Magical Congress of the United States of America, Elizabeth McGilliguddy presided over it. The pro-interventionalists argued that they could save lives by intervening and serve their country. The anti-interventionalists argued that it was not their fight and that they would be endangering their own kind by potentially exposing their magic.
When they did not reach any consensus, McGilliguddy asked the British Ministry of Magic whether they were actually planning to intervene and help their side. They replied with a simple four word letter stating that they were "sitting this one out". McGilliguddy replied with an even shorter letter stating "mind you do".
Neither side played any part in the war. However, the American witches and wizards did unofficially help No-Majs when needed and held their own celebrations for Independence Day when they won the war.