As for Pearl Harbor, even if one referred to it as a battle, it did not wipe out almost a fourth of the standing army, neither did the battle of Little Big Horn. However, the battle of Little Big Horn is on the right track so to speak. It is the battle I had in mind when I mentioned battles much more thoroughly documented in spite of the lower number of casualties.
The number of people killed in the battle we are looking for is about three times as high as the number of people killed in the battle of Little Big Horn.
Keep in mind that the smaller the standing army is, the "easier" it is to have a casualty rate of almost a quarter (this says something about the time in which the battle took place). It is really a US battle, so it took place after the war of independence.