First Dawn

How do you win a war lost 6,000 years before you were born? Launa O'Brian, fresh out of West Point, gets those for her first set of orders. They want her to help peaceful farmers win a war they're about to lose to horse raiders, and change all history. Since a designer plague has canceled all human history in the 21st century, refusing the orders is not an option.
The Army has a time machine. They tested it once. It didn't work, but trust them, they've fixed it. Yeah, Right.
With all the army to chose from, Launa has one subordinate, Captain Jack Walking Bear. Being a Captain, he outranks her. Being a guy, he's got definite opinions on how to run this show.
But the President's old anthropology professor says women spoke first back then, and Kris gets the command.
So how come the first thing they see is a war band of horsemen who just want to hit them over the head and steal everything they brought?
Did the time machine blow it?
Where the anthropologists a bit off?
Is Launa going to kill Jack before you get to the end of the book?
You really want to read this book.