No way to tell, depends on where you live too. If you found it in a place close to known battlefields there is a better chance of it being military. If you found it in a place like where I live, where no military activity ever happened, chances are very likely that it is not military. Your profile says you live in Washington, which did not have much military activity. In that case, It is unlikely that the buckle you found has to do with the military. Though there's a chance that a soldier from Washington could have fought overseas in a war, and brought back some clothes or other objects from the war to his hometown and may have lost a buckle. This is very unlikely though, and I would conclude that this buckle is probably civilian, not sure what it is from or a certain age too. It could be horse related, or like said above, dog related.
Anyways, cool find nonetheless, I love finding stuff like that and wondering the story behind it. It may not seem like much, but there is history behind every nail that is found