The military history of the United States dates back more than 300 years, if you include colonial times. Almost all of those conflicts were with the British, the French, the Spanish, or the Native Americans. The main exception was, of course, the Civil War. With each new conflict, American soldiers became better fighters, thanks to both training and technology. They started out using primarily hunting and wilderness survival tools. However, by World War II they were using sophisticated weaponry, aircraft, ships, and submarines.
It’s clear that military records can be useful for tracking ancestors who were in the military. However, they can also be useful for finding information about ancestors who were not in the military at all. That’s because military records for close relatives, such as fathers, sons, or brothers, can lead to information about the ancestor in question. Not only that, but military records may list wives, widows, and children of the person who served, which can allow the researcher to pinpoint multiple family members and their relationships to each other. Some military records are also useful purely for their historic value, since they may describe specific military battles, locations, or procedures. So, they should never be overlooked by researchers.
There are at least some records still available from every war involving the United States or the colonies. However, many records are damaged or missing. Also, the records were not always recorded in a uniform way.