Why did Robert E. Lee describe John Brown as an enemy of the country?

Robert E. Lee described John Brown as an "enemy of the Country" following Brown's raid on Harpers Ferry in October 1859.1

Lee, then a colonel in the U.S. Army, was summoned by the War Department to suppress the insurrection. Brown and his followers had seized the federal armory at Harpers Ferry, Virginia, as part of an effort to incite an uprising against the institution of slavery. Lee orchestrated the successful capture of Brown and his collaborators, who were subsequently indicted for treason against the state of Virginia.12

From the perspective of the federal military authority Lee represented at the time, Brown’s armed assault on a government facility and his attempt to destabilize the existing social and political order were viewed as direct acts of hostility against the United States.12