The North, bent not simply on invasion but on reconquest of the States which had seceded, necessarily took the offensive; the South stood mostly on the defensive. Yet while the population in the North was far in excess of that in the South, the initial advantages from a transport point of view were in favour of the South, which found its principal ally in the railways. Generals in the North are, indeed, said to have been exceedingly chary, at first, in getting far away from the magazines they depended on for their supplies; though this uneasiness wore off in proportion as organised effort showed how successfully the lines of rail communication could be defended. In these and other circumstances, and especially in view of the paramount importance the railway system was to assume in the conduct of the war, the Federal Government took possession of the Philadelphia, Wilmington and Baltimore Railway on March 31, 1861. This preliminary measure was followed by the passing, in January, 1862, by the United States House of Representatives, of "An Act to authorise the President of the United States in certain cases to take possession of railroad and telegraph lines, and for other purposes." The President, "when in his judgment the public safety may require it," was "to take possession of any or all the telegraph lines in the United States; ... to take possession of any or all the railroad lines in the United States, their rolling stock, their offices, shops, buildings and all their appendages and appurtenances; to prescribe rules and regulations for the holding, using, and maintaining of the aforesaid telegraph and railroad lines, and to extend, repair and complete the same in the manner most conducive to the safety and interest of the Government; to place under military control all the officers, agents and employés belonging to the telegraph and railroad lines thus taken possession of by the President, so that they shall be considered as a post road and a part of the military establishment of the United States, subject to all the restrictions imposed by the Rules and Articles of War." Commissioners were to be appointed to assess and determine the damages suffered, or the compensation to which any railroad or telegraph company might be entitled by reason of such seizure of their property; and it was further enacted "that the transportation of troops, munitions of war, equipments, military property and stores, throughout the United States, should be under the immediate control and supervision of the Secretary of War and such agents as he might appoint."