Why didn't secretary of war Armstrong leave troops to defend washintington dc?

Secretary of War Edwin M. Stanton did not leave troops to defend Washington, D.C., primarily due to the belief that the city was secure and that resources were needed on the front lines to support the Union Army. Additionally, there was a prevailing assumption that the Confederate forces were not capable of mounting a serious threat to the capital. This decision was influenced by the need to concentrate military efforts elsewhere, especially during critical battles in the Civil War. Ultimately, this judgment proved to be a miscalculation, as Washington faced significant threats during the war.