An alternate timeline is born as the United States of America is founded 144 years early (and on a different continent). History plays on around them.
The book divides its time between brutal battles fought across Germany, building a new society and some overlong passages on just how well the Americans are getting along with their new neighbors and each other. Putting a new government together has its growing pains. By the end of the book, you get a sense that things will go well even though there's much more to do and more dangers to face, but you'll be satisfied that you weren't left hanging until the next book.
1632 is the first of a series, but is a fine standalone read. It has launched a shared world of sorts, with other established authors writing stories. The first sequel will be on my "To Read" list before I decide how much farther I'll go with the series.