In the 1700s, a group of people in what is now the United States decided that their lives would be better if they didn't have to follow the rules of an overseas king. So, in 1776, they declared that they were free and independent, and would make their own laws. This was called the Declaration of Independence.
In 1776, the people in what was called the Thirteen Colonies formed a union and began to fight a war against Great Britain. This is known as the American Revolutionary War. The people of the Colonies wanted freedom from Britain and to make their own government.
In 1783, the Revolutionary War ended with the American colonies winning their independence. The colonies then formed the United States of America and drafted a new constitution. This was adopted in 1789. This new constitution described a system of government that respected the rights of all citizens and created a framework for the fifty United States to form a strong union.
The United States has grown from thirteen colonies to fifty states, but the same principles of freedom and democracy have been its guide since 1776.