On the night of May 12, following the Baltimore riot of 1861, the hill was occupied in the middle of the night by a thousand Union troops and a battery under the command of General Benjamin F. Butler, who had entered the city, under cover of darkness and during a thunderstorm, from Annapolis via the Baltimore &amp; Ohio Railroad. During the night, Butler and his men erected a small fort, with cannon pointing towards the central business district. Their goal was to guarantee the allegiance of the city and the state of Maryland to the United States government (with the implicit threat of force, should it have been necessary). This fort and the Union army presence persisted for the duration of the Civil War. A large flag, a few cannons, and a small Grand Army of the Republic monument remain to testify to this period of the hill's history.