Fighting a war can be exhausting, and your units will come away from each battle with less and less health. Naturally, this can't go on forever, and so now you must look into healing them.

There are two simple, easy ways to get your troops back into fighting shape.

Firstly, you can use Goldmark to restore a portion of their maximum health. This won't necessarily heal them back to full right away, and you may need multiple payments to fully heal a unit, but even a small boost to an army's health can make all the difference.

Secondly, units stationed on friendly territory will heal slowly over time, so if you position your land units in your own provinces, and your aircraft in their airfields, they will regenerate their health and morale.
Your Navy is a different situation, and will slowly heal over time while at sea.