Percentage-wise, King Philip's War remains one of the most deadly conflicts in US History, particularly in the colonial era. Plymouth Colony suffered at least 8% of its adult population killed in the war. Officially, the Civil War is percentage-wise the deadliest war after US became an independent nation. Approximately 2% of population died during the conflict from various causes. If that were applied to US's current population, that would be about 6 million people.