Andrew Johnson succeeded Abraham Lincoln.
"With the Assassination of Lincoln, the Presidency fell upon an old-fashioned southern Jacksonian Democrat of pronounced states' rights views. Although an honest and honorable man, Andrew Johnson was one of the most unfortunate of Presidents. Arrayed against him were the Radical Republicans in Congress, brilliantly led and ruthless in their tactics. Johnson was no match for them."
Andrew Johnson was Abraham Lincoln's vice president. When Lincoln, a Republican, was assassinated, Johnson, a Democrat, succeeded him to become the President of the United States during the Reconstruction period shortly after the American Civil War.
Johnson did not successfully lead the Reconstruction movement. Unfortunately for the South, Johnson’s attitude about Reconstruction maintained it as a state matter and not federal, unlike many of his Republican contemporaries.
"Johnson rose from poverty to a political career in state politics in Tennessee, which he represented in the U.S. Congress and where he was governor. Johnson championed Tennessee’s small white farmers and made enemies of the state’s large slaveowning planters. Staunchly Unionist, he was the only senator from a seceding state to remain in the Senate, and he served as Tennessee’s military governor during the war. His vice presidency, Republicans hoped, would gain support for the party in the South. But Johnson lacked Lincoln’s political acumen and ability to compromise. Johnson also defended states’ rights; he argued that since secession was illegal, southern states never actually left the Union or gave up their right to govern their own affairs. And Johnson, while supporting emancipation, was deeply racist, and did not believe blacks had a role to play in Reconstruction." - Foner, Eric. Give Me Liberty! An American History Fourth Edition. PowerPoint Chapter 15, 2015.
With the Assassination of Lincoln, the Presidency went to Andrew Johnson.
Many people in Congress also believed that Johnson would subscribe to their ideals since Johnson was raised in the South and abhorred the social hierarchy of the south in which Plantation owners controlled most of the land as well as the politics of the south. Congress wanted to see the South punished for their "disloyalty" to the Union and hoped that Johnson would back their Radical Reconstruction plans. These congressman were referred to as "Radical Republicans".
Johnson however, surprised these Radical Republicans by supporting Lincoln's more moderate reconstruction plan. Because of this, he made many enemies in congress and was the first president to be impeached by Congress. During his trial however, many members of Congress realized that Johnson wasn't being accused of "high crimes and misdemeanors" as defined by the Constitution. He was merely guilty of a differing of political opinion.