The secessionists possessed about two-thirds of each branch of the State legislature, and the general government had what it regarded as good reasons for believing that a secret, extra, and illegal session of the legislature was about to be convened at Frederick on the 17th of Sept. in order to pass an ordinance of secession. It was understood that this action was to be supported by an advance of the Southern army across the Potomac....It was impossible to permit the secession of Maryland, intervening, as it did, between the capital and the loyal States, and commanding all our lines of supply and reinforcement. I do not know how the government obtained the information on which they reached their conclusions. I do not know how reliable it was. I only know that at the time it seemed more than probable, and that ordinary prudence required that it should be regarded as certain. So that when I received the orders for the arrest of the most active members of the legislature, for the purpose of preventing the intended meeting and the passage of the act of secession, I gave that order a most full and hearty support as a measure of undoubted military necessity.
On the 10th of Sept. Hon. Simon Cameron, Secretary of War, instructed Gen. Banks to prevent the passage of any act of secession by the Maryland legislature, directing him to arrest all or any number of the members, if necessary, but in any event to do the work effectively.
On the same day the Secretary of War instructed Gen. Dix to arrest six conspicuous and active secessionists of Baltimore, three of whom were members of the legislature.