There are many historical events that have occurred over centuries. Some events, although significant, did not transform the USA, others  have completely changed its culture and nation, but from my point of view, the Declaration of Independence is one of the most significant event for the American people.

Source: https://en.wikipedia.org/wiki/United_States_Declaration_of_Independence

   The Declaration explained why the Thirteen Colonies at war with the Kingdom of Great Britain regarded themselves as thirteen independent sovereign states, no longer under British rule.

   With the Declaration, these new states took a collective first step toward forming the United States of America.

   After ratifying the text on July 4, Congress issued the Declaration of Independence in several forms. It was initially published as the printed Dunlap broadside that was widely distributed and read to the public.

   The signing of the Declaration of Independence was the official notice by the American colonies that they were willing to obtain their independence from England and they created their own future while Canada and Australia are still considered British commonwealths.