In an alternate history, the point of divergence (usually abbreviated POD or PoD) is a single event which happens differently from IOW, causing future events to take a different turn and creating an alternate timeline. In For Want of a Nail . . ., the POD is just before or during the Battle of Saratoga, bringing about a defeat of Horatio Gates' Patriot army by the British army of John Burgoyne. This leads to the defeat of the North American Rebellion, the establishment of the Confederation of North America, and the Wilderness Walk of defeated Patriots who then found the state of Jefferson and eventually the United States of Mexico.
There is a good argument to be made that no person conceived after the POD in IOW should exist as genetically the same person in an alternate timeline. Surely after a few years, circumstances would prevent an IOW person's parents from even meeting nearly all of the time, especially in such mobile societies as North America's or Mexico's. Yet Sobel has IOW people born all through the nineteenth century, for which we must grant him dramatic license.