You have given a lot of context, but not enough to allow me to figure out what this metaphor means. One page might not be enough. I might need to read two or three pages before this page to give you a good answer.
One thing that is clear is that Lincoln feels that he has run out of something. However, what that something is is not clear. He could be referring to a depletion of financial resources. He could be referring to a depletion of military supplies. He could be referring to a lack of soldiers.