"Up and at 'em!"???

Does anyone know the origin of the phrase
"up and at 'em"? Or more accurately "up and at them". This phrase is used when telling someone to get out of bed. Haven't we all used or heard this one before? I can't seem to find it anywhere. Any help would be hugely appreciated. Thanks.

Hey it's me again. Was wondering if anyone who planned on replying to this post would send whatever they have to say to my email address. I'll probably forget to check back here. Or, does this message board send an email automatically when someone replies to your post? Hmmm. . . Whatever the case, this is going to drive me insane if I don't figure it out. Please Help! :) thanks