' s ▁transition ▁to ▁a ▁volunte er ▁force . ▁Person nel ▁serving ▁in ▁the ▁United ▁States ▁Army ▁during ▁a ▁major ▁national ▁emer gency ▁or ▁armed ▁conflict ▁( e ither ▁volunt arily ▁or ▁invol unt arily ) ▁were ▁en list ed ▁into ▁the ▁Army ▁of ▁the ▁United ▁States , ▁without ▁specifying ▁service ▁in ▁a ▁component . ▁ ▁The ▁term ▁" Ar my ▁of ▁the ▁United ▁States " ▁or ▁" Arm ies ▁of ▁the ▁United ▁States " ▁is ▁also ▁the ▁legal ▁name ▁of ▁the ▁collect ive ▁land ▁forces ▁of ▁the ▁United ▁States , ▁as ▁pres cribed ▁by ▁the ▁United ▁States ▁Constitution . ▁ ▁In ▁this ▁concept , ▁the ▁term ▁" Ar my ▁of ▁the ▁United ▁States " ▁has ▁been ▁in ▁use ▁since ▁at ▁least ▁ 1 8 4 1 , ▁as ▁in ▁the ▁title ▁General ▁Reg ulations ▁for ▁the ▁Army ▁of ▁the ▁United ▁States . ▁ ▁History ▁ ▁World ▁War ▁I ▁The ▁original ▁concept ▁of ▁a ▁non - Reg ular ▁Army ▁component , ▁existing ▁to ▁augment ▁the ▁standing ▁military , ▁can ▁trace ▁its ▁orig ins ▁to ▁the ▁United ▁States ▁Vol unte ers . ▁State ▁volunte er ▁forces ▁were ▁used ▁extens ively ▁to ▁augment ▁the ▁Reg ular ▁Army ▁throughout ▁the ▁ 1 9 th ▁and ▁early ▁ 2 0 th ▁century . ▁ ▁During ▁World ▁War ▁I ▁and ▁dict ated ▁by ▁the ▁prov isions ▁of ▁the ▁National ▁Def ense ▁Act ▁of ▁ 1 9 1 6 , ▁states ▁contributed ▁men ▁to ▁the ▁" Vol unte er ▁Army " ▁( more ▁commonly ▁known ▁as ▁the ▁National ▁Army