I previously detailed how men were assigned to the services and where they served in posts here and here, but more answers can be found on my profile here.

The majority of the Army's effort was directed at the combined European and Mediterranean Theaters, and essentially the entirety of the Marine Corps and most of the Navy spent the entire war or portions of the war in the Pacific.

30 April 1945 Reported Actual Strengths in Principal Theaters (U.S. Army)

30 April 1945	ETO	MTO	SWPA	POA	CBI	Combined	Total Army
Divisions	819,342	102,485	183,798	80,834		1,186,459
Non-divisional Combat Troops	595,418	37,797	113,318	102,076	15,766	864,375
Total Ground Combat Troops	1,414,660	140,282	297,118	182,910	15,766	2,050,835
Non-divisional Service Troops	828,726	128,307	214,835	142,579	69,898	1,384,345
Air Service	439,425	153,005	173,343	75,438	90,949	932,160
Replacements	171,933	34,551	34,203	20,371	4,355	265,423
Patients in SOS Hospitals	128,305	15,679	41,135	6,423	1,439	192,981
Unclassified	27,598	4,972	29,500	19,851	10,447	131,636
Total	3,065,505	493,876	802,504	450,952	198,830	5,011,657	8,290,993