During the civil war, southern spies would live in washington D.C. and tell the south about the northerners plans.
One southern spy was named Rose O'Neal Greenhow. She lived on a farm and acted like she supported the north. She would overhear the battle plans and contact the south about them.
The spies, after they learned what they needed from the north. Would go and tell the south what was going on. They would normally meet in the south and try and take as little time as possible.
Correlation to modern day: The tactics used by the southern spy system are the starting for modern spies.
Unfortunately, the spy system in the south had hurt the north in its attempts to reunite the nation. If it was not for the spies in the south, the north would more than likily won the first battle of bull run.
Fun Fact: Union soldiers did not have an organised spy system like the south did. instead, the generals would look and become the spies, even if they were very unorganized.