What is the true beginning of USA?

The thing is that the people we call the "Founding Fathers" were not the founding fathers of the United States (or at least the United States in a cultural sense), they were the founding fathers grandsons, and great grandsons. Before the United States existed as a political Union there were the 13 British colonies and they were founded by very different people for very different reasons and created distinct cultures whose values were and are mutually exclusive.

The Puritans who came to Massachusetts to found a utopian Calvinist theocracy in the American wilderness had nothing in common with with the Caribbean slave lords that founded my state of South Carolina . Different parts of the USA were settled first by different groups of people creating cultures that are more different in values than any two European states. A person from France and Germany may speak different languages and their ancestors may have been at war with each other many times but they would agree on much more politically, religiously, behaviorally, and morally with one another than someone from Vermont and someone from Mississippi would.

These different cultures came together under one government after the Revolution only because they feared that if the colonies stayed separate and didn't form some kind of protective Union with each other the empires of France, Spain, and Britain would conquer them. The Union was formed then as it stays together now for purposes of self defense.

Originally the Union however was created with decentralization in mind and the United States did not consider itself as a unitary nation. Each colony (now called states) were guaranteed self determination by the tenth amendment of the Constitution. If you had asked the average person before the Civil War what country they came from they would say, "Massachusetts, New York, South Carolina, Virgina, Kentucky," and so forth. Their state was their country in their mind and the idea of America as a nation had not yet formed. This changed after the Civil War when Yankee nationalism overcame Southern provincialism and Puritan New England forced its identity on the whole of the rest of the country.

In any event these different cultures, these "American Nations" still exist today and American unity has always been somewhat of an illusion. Today we are seeing that American national identity formed with the ascendancy of the Yankees, (and by Yankees I mean the culture and people of those areas first settled by Puritans and the descendants of Puritans heading west across the continent, New England, Upstate New York, the Upper Midwest, and the West Coast, not all Northerners) falling apart around us.

So regarding your question of what the true beginning of the USA is, I would say that the true beginning was when the English first planted a permanent colony in Virginia.