Robert Chevalier de la Salle, the French historian Anka Muhlstein writes, stands indisputably among Europe's most accomplished explorers. In the 1670s and '80s he crossed over most of what is now French Canada and the middle United States and charted the length of the Mississippi River--almost always traveling on foot, almost always under dangerous circumstances. Thanks to his pioneering travels, in the next century "America seemed destined to be...