It is a common misconception that World War 2 was the deadliest war for the United States. However, most people do not realize that more American soldiers died (Union & Confederate) during the Civil War than in every war before and after added together including WW 2.
About 3,867,500 men were enrolled in the Union and Confederate Armies during the 48 months of the War Between the States (American Civil War). Of these approximately 184,594 were killed in combat and 373,458 from other causes or as prisoners of war (558,052 total). That averages out to about 11,627 deaths per month. The average soldier/sailor had a 14.4% chance of dying during the course of the war.
About 16,353,700 men were enrolled in the American military during the 44 months of World War 2. Of these approximately 292,131 were killed in combat and 115,185 from other causes or as prisoners of war (407,316 total). That averages out to 9257 deaths per month. The average soldier/sailor/airman had a 2.5% chance of dying during the course of the war.