Why did general butler declare slaves to be contraband?

General Benjamin Butler declared slaves to be contraband of war during the American Civil War to prevent them from being returned to their Confederate owners. He argued that since the Confederate states were in rebellion against the Union, their property, including slaves, could be seized as a military necessity. This declaration helped to provide refuge for escaped slaves and laid the groundwork for the eventual emancipation policies adopted by the Union. It also marked a significant shift in the Union's approach to the issue of slavery during the war.