Sorry for the lack of posts recently, but I've had company non-stop for the last few weeks. Yesterday I was talking to my mom and we started talking about how you need to be knowledgeable about history in order to succeed in tracing your ancestry. I decided to expand on our conversation in a blog post here on Free Genealogy Resources in the hopes a reader might find it helpful.
If you know when wars were fought, you can determine which ancestors lived during that period and might have served. This opens the door to finding enlistment records, muster rolls or discharge papers, all of which can yield information on not only your ancestor, but also their family. After the completion of service, there may be additional records to be found in pension applications by the veteran or his widow. If you didn't realize that a war had occurred during your family's history, you might miss all of these records.
It also helps to be aware of health epidemics that occurred during history. If an ancestor disappeared from all records during certain periods of time, it's possible they fell victim to smallpox, yellow fever, cholera or one of the other diseases that struck through the years. For instance, the 1918-1919 influenza pandemic struck world-wide, killing more people than World War I did. BJ's Genealogy Site has a nice list of epidemics that occurred 1628-1918, including ones that were only in specific regions of the US.
Land boundaries also change over time. Many disappearing ancestors didn't actually go anywhere between censuses, the name of their location may have changed. Knowing historic boundaries can be helpful when tracing family through the years. If I didn't know that Cook County, Georgia was formed from Berrien County in 1918, which was formed from pieces of the counties of Lowndes, Coffee and Irwin in 1856, I would have had trouble tracing my maternal line in Georgia. The Atlas of Historical County Boundaries can be a helpful resource for seeing how lines changed through time.
Knowing about disasters can be helpful as well. A fire, earthquake, flood, hurricane, tornado or other event can be the reason behind an ancestor's death or the family's move from an area. There is a nice table of US Disasters on Wikipedia. It's not sorted by year, but is still worth taking a look through to see if you can link one to someone in your family tree.