We would not go to war with China because we owe them WAY too much money, they would basically break our economy in one move.
If we fought a war with Russia it would probably go nuclear very quickly so there would not be a need for an excess of soldiers.
There is no draft so the point is pretty moot. The only past precedent you can go off of is the draft back in the day and even for Vietnam anyone enrolled in a collegiate program was exempt, as well as those in medical school. Plus women. Plus if you were the only son.
Here's a list I found, I think it was accurate.
PWWII period: (1948 thru 1976*)
1-A Available for military service
1-AM Medical specialist available for military service
1-A-O Conscientious objector available for noncombatant
military service
1-A-OM Medical specialist conscientious objector available
for noncombatant military service
1-C Member of the Armed Forces of the United States, the
Coast and Geodetic Survey, or the Public Health Service
(Enl)-enlisted; (Ind)-Inducted; (Dis)-discharged
1-D Member of a Reserve component or student taking military
training
**1-H Registrant not currently subject to processing for
induction or alternate service
1-O Conscientious objector available for civilian work
contributing to the national health, safety, or interest
1-OM Medical specialist conscientious objector available for
civilian work contributing to the national health,
safety, or interest
1-S Student deferred by statute: (H)-high school; (C)-college
1-W Conscientious objector performing civilian work in the
national health, safety, or interest (Rel)-released
***1-Y Registrant qualified for service only in time of war
or national emergency
2-A Registrant deferred because of civilian occupation
(except agriculture)
2-AM Medical specialist deferred because of critical
community need involving patient care
2-C Registrant deferred because of agricultural occupation
2-D Registrant deferred because of study in preparation
for the ministry
2-M Registrant deferred for medical study
2-S Registrant deferred because of activity in study
3-A Registrant with a child or children; or registrant
deferred by reason of extreme hardship to dependents
4-A Registrant who has completed service; or sole surviving son
4-B Official deferred by law
4-C Alien
4-D Minister of religion or divinity student
4-E Conscientious objector opposed to both combatant and
noncombatant training and service
4-F Registrant not qualified for military service
4-FM Medical specialist not qualified for military service
4-G Sole surviving son
4-W Conscientious objector who has completed civilian
alternate service
5-A Registrant over the age of liability for military service
