Why were enslaved people who reached Union lines called contrabands?

During the American Civil War, enslaved people who fled to Union-controlled territory were labeled "contrabands" as a legal strategy to secure their freedom. In May 1861, General Benjamin F. Butler refused to return three fugitives who had reached his lines at Fort Monroe, Virginia. He argued that because these individuals had been used to support the Confederate war effort, they could be legally classified as "contraband of war"—or confiscated enemy property—and therefore did not have to be returned to their enslavers under the Fugitive Slave Acts.12

This interpretation was eventually adopted as official federal policy, effectively emancipating those who reached Union lines. This classification provided a crucial pathway to freedom for over 500,000 people, who often resided in "contraband camps" where they contributed to the Union war effort and began transitioning to independence.12