Image of Baltimore from Federal Hill via Wikipedia
On Monday, May 13, 1861, General Benjamin Butler occupied Baltimore, stamping out secessionist activity there and securing it for the Union for the duration of the war.
Butler had occupied Relay, Maryland, just south of Baltimore, on May 5. From there, he had reported back to the War Department, tales of military stores and gunpowder being sent to the Rebels. He received some vague verbal orders; he was expected to make a few arrests and seize the military supplies. He used those orders to occupy Baltimore and institute martial law there.
Butler marched his men into Baltimore on May 13, placing artillery on Federal Hill, commanding the business district and the harbor. Butler then issued a proclamation, declaring the military in charge. Any display of Southern sympathy was quickly squelched and dozens of arrests were made. Suspected pro-Southerners were imprisoned at Fort McHenry with no charges filed.
General-in-chief Winfield Scott was livid that Butler had exceeded his orders. He was especially concerned that Butler might have touched off another riot in the city. Scott would remove Butler from command, but Lincoln would promote him to major general and send him to Fort Monroe, a quieter post where he wouldn't cause so much trouble.
But although Butler was gone, martial law would remain. The arrests would continue, and Baltimore would be secured, opening the Baltimore & Ohio Railroad to Washington again.