26 CFR 31.3401(a)(1)-1 - Remuneration of members of the Armed Forces of the United States for active service in combat zone or while hospitalized as a result of such service.
Remuneration paid for active service as a member of the Armed Forces of the United States performed in a month during any part of which such member served in a combat zone (as determined under section 112) or is hospitalized at any place as a result of wounds, disease, or injury incurred while serving in such a combat zone is excepted from wages and is, therefore, not subject to withholding. The exception with respect to hospitalization is applicable, however, only if during all of such month there are combatant activities in some combat zone (as determined under section 112). See § 1. 112-1 of this chapter (Income Tax Regulations).