It seems to be relatively recent. The OED doesn’t list draw down specifically for this sense (the more usual sense means to invoke, to pull in, as in to draw down the wrath of God). Although there is this early 19th century cookery citation, meaning to boil down, to reduce:
1806 Culina 15 Put all those into a stew pan, with some water, and draw them down to a light brown colour.
The Big Dic does list a military sense meaning to withdraw or move troops for draw off, dating to the 17th century. Not quite the same sense as it doesn’t convey reduction in numbers though:
1667 MILTON P.L. IV. 782 Half these draw off. 1697 W. DAMPIER Voy. I. iv. 84 Captain W. drew off his men. 1736 T. LEDIARD Life Marlborough I. 377 He resolved to draw off his Dragoons.