What is the word that means ready to step into battle whenever necessary? It's used to describe someone often in the military on standby basically.
I believe you are referring to a "military reserve force", or often spoken as "military reserves". Those are the people who are trained as military, but only get called up for active duty if there is a need.
If there are military forces being added to an existing military operation -- either an ongoing battle, operation, occupation, etc., those are "reinforcements".