This is a book about how the United States was created during and, more importantly, after the Revolutionary War. Ellis's main point is that there were different views of how the new country should be set up and that the founding generation went through a series of arguments over which of these views should prevail.
The major conflict in the book is between the Jeffersonian view of what the country should be and the Hamiltonian view. Jefferson's view was more democratic and antifederalist. Hamilton's was more republican and federalist. The two factions came into conflict time and again as the country was being created.
This book is about these conflicts between the "founding brothers" with regard to what sort of country the US should become. It is meant to help us understand the very deep roots of some of the issues we face today.