Espionage had a crucial impact on the Civil War for both the Union and Confederacy, as spies on both sides helped gather vital information about enemy positions and plans, allowing them to base their own strategies based on this information, signficantly influencing the outcome of the war.
During the Civil War, neither the Union nor the Confederacy had an official centralized military intelligence agency, but both sides still had unofficial spy organizations that gathered crucial information throughout the war. For example, the Confederacy organized a spy network in Washington, home to numerous Southern sympathizers. Union generals often organized spy operations themselves, as the Union had no central intelligence network (though there were a few small unofficial spy organizations). In addition, being a spy was very risky, as spies and saboteurs who were discovered were considered traitors and often executed. This also led to them being seen as legendary and patriotic individuals by those who utilized their information, mostly generals and other military figures, as they were putting their lives on the line to gather crucial information. Also, although both sides employed espionage, the Union had a slight disadvantage to the Confederacy, as while the Union did have an organized government with branches that Confederate spies could infiltrate, the Confederacy had no government, making it harder for Union spies to gather vital intel. Despite these circumstances aiding them, the Confederates were not successful in gaining a significant advantage in terms of espionage.
Although espionage was often conducted through building personal connections with important figures who held positions in government offices or had access to war plans, lots of important information could also be found in newspapers or by talking to everyday citizens.
Also, women made great spies due to the sexist stereotypes of the time. It was hard for people to imagine that women were spies and had the ability to make serious contributions to the war, so, even if they were caught, their punishments were not as severe as those of male spies.