<P> On May 13, 1861 General Benjamin F. Butler entered Baltimore by rail with 1,000 Federal soldiers and, under cover of a thunderstorm, quietly took possession of Federal Hill . Butler fortified his position and trained his guns upon the city, threatening its destruction . Butler then sent a letter to the commander of Fort McHenry: </P> <P> I have taken possession of Baltimore . My troops are on Federal Hill, which I can hold with the aid of my artillery . If I am attacked to - night, please open upon Monument Square with your mortars . </P> <P> Butler went on to occupy Baltimore and declared martial law, ostensibly to prevent secession, although Maryland had voted solidly (53 - 13) against secession two weeks earlier, but more immediately to allow war to be made on the South without hindrance from the state of Maryland, which had also voted to close its rail lines to Northern troops, so as to avoid involvement in a war against its southern neighbors . By May 21 there was no need to send further troops . After the occupation of the city, Union troops were garrisoned throughout the state . By late summer Maryland was firmly in the hands of Union soldiers . Arrests of Confederate sympathizers and those critical of Lincoln and the war soon followed, and Steuart's brother, the militia general George H. Steuart, fled to Charlottesville, Virginia, after which much of his family's property was confiscated by the Federal Government . Civil authority in Baltimore was swiftly withdrawn from all those who had not been steadfastly in favor of the Federal Government's emergency measures . </P> <P> During this period in spring 1861, Baltimore Mayor Brown, the city council, the police commissioner, and the entire Board of Police, were arrested and imprisoned at Fort McHenry without charges . One of those arrested was militia captain John Merryman, who was held without trial in defiance of a writ of habeas corpus on May 25, sparking the case of Ex parte Merryman, heard just 2 days later on May 27 and 28 . In this case U.S. Supreme Court Chief Justice, and native Marylander, Roger B. Taney, acting as a federal circuit court judge, ruled that the arrest of Merryman was unconstitutional without Congressional authorization, which Lincoln could not then secure: </P>

What was the purpose of martial law in maryland
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