Before the purchase of the Louisiana Territory in 1803, Mason and Dixon Line and the Ohio River formed the dividing line between the free States on the north and the slave States on the south. But after that purchase there was a prolonged struggle to determine whether the new territory should be slave or free. It was thought that the Missouri Compromise of 1820 would forever settle the trouble, but such was not the case. It broke out again, as bitter as ever, about the Mexican Cession, which became ours as a result of the Mexican War. Again it was hoped that the Compromise of 1850 would bring an end to the struggle. But even after this second compromise, the agitation over slavery continued to become more and more bitter until Mr. Lincoln's election, when some of the Southern States threatened to secede, that is, withdraw from the Union. These States claimed the right to decide for themselves whether or not they should remain in the Union. On the other hand, the North declared that no State could secede from the Union without the consent of the other States. Before Lincoln was inaugurated, seven of the Southern States had seceded. The excitement was everywhere intense. Many people felt that a man of larger experience than Lincoln should now be at the head of the Government. They doubted the ability of this plain man of the people, this awkward backwoodsman, to lead the destinies of the nation in these hours when delicate and intricate diplomacy was needed. But, little as they knew it, he was well fitted for the work that lay before him. While on his way to Washington for inauguration, his friends learned of a plot to assassinate him when he should pass through Baltimore. To save him from violence, therefore, they prevailed upon him to change his route and make the last part of his journey in secret. In a few weeks the Civil War had begun. We cannot here pause for full accounts of all Lincoln's trials and difficulties during this fearful struggle that began in 1861 and ended in 1865. His burdens were almost overwhelming, but, like Washington, he believed that "right makes might" and must prevail. When he became President he declared that the Constitution gave him no power to interfere with slavery in the States where it existed. But as the war continued, he became certain that the slaves, by remaining on the plantations and producing food for the Southern soldiers, were a great aid to the Southern cause, and thus threatened the Union. He therefore determined, as commander-in-chief of the Union armies, to set the slaves free in all territory whose people were fighting against the Union. He took this step as a military necessity.