The US Army, including the Army Air Forces, lost more men than any other US service branch in WWII. The Navy and the Marine Corps followed, each with losses in the tens of thousands. The Army accounted for well over half of all US military deaths in the war.
Rough totals
The total for US military deaths in WWII is most often cited as around 405,000, based on Defense Department figures. Other tallies differ, sometimes by thousands, depending on what is counted, such as combat deaths, deaths from disease and accidents, and wounds that proved fatal later. Treat any single figure as an estimate, and check how the source defines its count before comparing it with another.
The Army, Navy and Marine Corps
The Army had the largest share by far. Its losses came in Europe, North Africa and the Pacific, and the Army Air Forces are counted within the Army total. The Navy suffered heavy losses at sea, particularly in the Pacific. The Marine Corps lost many men in island campaigns. The Coast Guard’s losses were much smaller than the other three.
Why the numbers differ between sources
Sources differ on what they count, on how they treat deaths that happened after the war from wounds, and on how they split the figures into categories. A branch-by-branch figure from one source should not be combined with a total from another without checking both definitions.
Where to check
The Defense Department’s casualty figures and the National WWII Museum are reasonable starting points. Check the publication date and the definitions before you quote a number. If you are researching one man who was killed or wounded, a surviving Purple Heart and his dog tags can help connect the records to a name.