Were new American soldiers allowed to to choose which theater they would be shipped to? Was fighting against Germany and Hitler considered more of a righteous cause than fighting the Japanese? Or vice versa?
Imagine that you just enlisted in your local enlistment office and they told you they would have your orders in a few days. During that waiting period are you hoping for the Pacific or for Europe?
At the level of the newly enlisted man, the answer was generally no, as the needs of the military often pre-empted most individual choices even in cases where volunteering was nominally possible.
The lion's share of the Army's effort was in Europe, and the vast majority of the Navy and Marine Corps spent the war or portions of the war in the Pacific. Assignment to a particular theater was generally dependent upon higher unit assignment, which was determined by the needs of the military itself. Aptitude in a specific task could result in assignment to a particular type of unit, and certain types of units were predominantly deployed to only certain theaters, resulting in a higher likelihood of being assigned to the theater in question.
Voluntary enlistment was mostly discontinued on December 5, 1942 via Executive Order 9279
30 April 1945 Reported Actual Strengths in Principal Theaters
| 30 April 1945 | ETO | MTO | ETO+MTO | SWPA | POA | CBI | Combined | Total Army |
|---|---|---|---|---|---|---|---|---|
| Divisions | 819,342 | 102,485 | 921,827 | 183,798 | 80,834 | 1,186,459 | ||
| Non-divisional Combat Troops | 595,418 | 37,797 | 633,215 | 113,318 | 102,076 | 15,766 | 864,375 | |
| Total Ground Combat Troops | 1,414,660 | 140,282 | 1,555,042 | 297,118 | 182,910 | 15,766 | 2,050,835 | |
| Non-divisional Service Troops | 828,726 | 128,307 | 957,033 | 214,835 | 142,579 | 69,898 | 1,384,345 | |
| Air Service | 439,425 | 153,005 | 592,430 | 173,343 | 75,438 | 90,949 | 932,160 | |
| Replacements | 171,933 | 34,551 | 206,484 | 34,203 | 20,371 | 4,355 | 265,423 | |
| Patients in SOS Hospitals | 128,305 | 15,679 | 143,984 | 41,135 | 6,423 | 1,439 | 192,981 | |
| Unclassified | 27,598 | 4,972 | 32,570 | 29,500 | 19,851 | 10,447 | 131,636 | |
| Total | 3,065,505 | 493,876 | 3,559,381 | 802,504 | 450,952 | 198,830 | 5,011,657 | 8,290,993 |
Interesting situations could result where, when during the activation process of a unit, numbers of men who voluntarily enlisted or were drafted into the Army at a certain time from the same region could end up being assigned to the same unit and eventually serving in the same theater. To economize activation or expansion of new units, the Army would often send numbers of fillers from Service Command reception centers near where a unit was to begin training.
| Service Command | Headquarters | States |
|---|---|---|
| 1 | Boston, MA | CT, ME, MA, NH, RI, VT |
| 2 | Governors Island, NY | DE, NJ, NY |
| 3 | Baltimore, MD | DC, MD, VA, PA |
| 4 | Atlanta, GA | AL, FL, GA, MS, NC, SC, TN |
| 5 | Columbus, OH | IN, OH, KY, WV |
| 6 | Chicago, IL | IL, MI, WI |
| 7 | Omaha, NE | CO, IA, KS, MN, MO, NE, ND, SD, WY |
| 8 | Dallas, TX | AR, LA, NM, OK, TX |
| 9 | Fort Douglas, UT | AZ, CA, ID, MT, NV, OR, UT, WA |
The 35th Infantry Division (National Guard from Kansas, Nebraska, and Missouri) received most of its fillers from the Seventh Service Command in early 1941 after it moved to Camp Joseph T. Robinson, Arkansas, and this is still in evidence in its subordinate units over three years later in May 1944 as the division prepared to ship to Europe. The original "2072" men (federalized Nebraska National Guard personnel) were swimming in a proverbial sea of "17" and "37" men and men from other Service Commands, soon to be diluted even more as loss replacements drafted from all over the country flowed in beginning in mid-July 1944.
The assignment of units to theaters also depended upon when the unit was activated, when it had completed its training, where it had done such training, and the overall strategic situation.
Men assigned as loss replacements, the main focus in training in the ground and air Army after 1943, constituted about two-thirds of newly-inducted Army personnel by the summer of 1944. As most replacement training centers were in the lower Great Plains or southeastern United States to facilitate year-round training, they were filled with men from all over the country.
I speak at significant length about the acquisition and training of manpower for the U.S. Army during World War II in these other answers;
Why were injured soldiers that had recovered sent back to different units in WW2?
[American manpower situation during WWII] (https://www.reddit.com/r/AskHistorians/comments/4yrj2r/ww2_ive_heard_contradicting_reports_on_how/?)
Sources:
Palmer, Robert W. The Army Ground Forces: The Mobilization of the Ground Army, Study no. 4. Washington: Historical Section, Army Ground Forces, 1946.
As howling cow notes, the Marines fought exclusively in the Pacific (well almost exclusively, as I think some individual Marines served as liasons or observers or in the OSS and such in Europe, but no actual combat units that I'm aware of). Before voluntary enlistment ended, someone who wanted to fight the Japanese could enlist in the Marines and thereby essentially choose the Pacific theater. I seem to recall reading memoirs where this conscious choice was made, although I couldn't give you a name now.