What unites the people of the United States of America is not race. It is not a common religion. It is a set of values and ideals.
The United States of America began its journey as 13 British colonies in North America. In 1776, they decided to fight against the tyranny of British rule and for their independence. It was an unequal battle between a well-trained professional British army and a ragtag army consisting mostly of volunteers from the 13 colonies. The British should have won but they lost to a more determined enemy, which had the support of France.