There were really a number of things that lead to Andrew Johnson’s impeachment, though the dismissal of Secretary of War, Edwin M. Stanton was the tipping point that sent the impeachment into motion.
It’s important to note that while he was a man of the Union, Johnson’s was born and raised in North Carolina—a Southern state and eventual part of the Confederacy. He sympathized with the Southerners when it came to slavery, but he believed their wishes to secede were extreme and that's what drove him to side with the Union. At the beginning of the Civil War, he was very anti-emancipation, only changing his mind when he figured that freeing the slaves would be a harsh blow to the Confederacy, taking away important resources from them as punishment.
The Civil War ended on May 9, 1865 and President Lincoln was assassinated shortly after this on April 14, placing then Vice President Johnson into the presidency and thus, in charge of post-Civil War reconstruction. He was a staunch opponent of equality for black people and during a Congressional Recess, he started putting to work his own ideas for reconstruction which included looking the other way while the South established “black codes” which stripped African-Americans of their newly earned rights and kept them as a source of cheap labor. Johnson also pardoned a great deal of former confederates.
This infuriated a lot of Radical Republicans and when Congress reconvened, they quickly worked to fix the problems Johnson had started to cause by passing the following:
Freedmen’s Bureau Bill: this was actually created under the Lincoln administration in 1865. It provided food and shelter, education, employment contracts, and health care to former slaves. A follow up/renewal of this bill was vetoed by President Johnson in 1866. Because congress failed to override the veto, it had to go through legislation again before it was eventually passed.
The Civil Rights Act of 1866 & Fourteenth Amendment: this granted all citizens of the US equal rights under the law, including African-Americans. Johnson vetoed this but congress overruled it.
Reconstruction Acts of 1867: these were designed to divide the South into five military districts, each headed by a general. The goal was to ensure that the South was upholding the new laws. Johnson vetoed this of course, stating that “the measure proposed by this bill does violate the Constitution”. Yet again, his veto was overruled.
Tenure of Office Act of 1867: what would eventually lead to Johnson’s downfall, this act prohibited presidents from being able to remove cabinet members without support from the senate. This was also vetoed by Johnson and overruled by congress.
While this was happening, Johnson was growing desperate and humiliated. He went on a series of drunken tirades, alienating and insulting Republicans and congress in a terrible attempt to try and get the people to back him up. They didn’t of course and in a last ditch effort to try and assert his power, he outright defied Congress and the Tenure Office Act by removing the Secretary of War, Edwin M. Stanton, from office without the approval of the senate. His reason behind this was that Stanton was a Radical Republican and he knew that he would not go along with his ideas for Reconstruction.
This didn’t happen all at once though. The Tenure of Office Act did allow presidents to suspend executive officers during recess with the intention of having the dismissal finalized or rejected upon their return from congress. So Johnson suspended Stanton and appointed Ulysses S. Grant to fill the position in the meantime.
On January 7, 1868, after congress had reconvened, the senate denied his wishes to have Stanton dismissed permanently. Johnson did not let that sway him and carried on with Grant as his Secretary of War until Grant resigned on January 28, stating his dislike of the politics and drama that Johnson had roped him into. Johnson replaced him with Lorenzo Thomas on February 21, 1868 and had Thomas deliver the note of dismissal to Stanton himself. Stanton holed himself up in his office, proclaiming that what Johnson was doing outright violated the Tenure of Office and called for the arrest of Thomas.
On February 24, the House of Representatives impeached Johnson by a vote of 126 to 47. There were actually 11 articles of impeachment which you can find the exact transcript of here.
Johnson avoided removal of office by a single vote. He continued to veto Reconstruction acts throughout the remainder of his term and congress continued overruling them.
So it was really a downward spiral of events that lead to Johnson’s eventual impeachment, but what drove the nail in the coffin was his attempted removal of Secretary of War, Stanton.
SOURCES
Andrew Johnson: A Biography by Hans Trefousse WorldCat
Impeachment of a President by Hans Trefousse WorldCat
Law Creating the Freedmen's Bureau read online!
Fourteenth Amendment to the US Constitution read online!
An Act to provide for the more efficient Government of the Rebel States read online!
Veto for the First Reconstruction Act March 2 1897 by Andrew Johnson read online!
Tenure of Office Act read online!
Proceedings of the Senate Sitting for the Trial of Andrew Johnson read online!
Edit; formating ugh
Edit 2; added links to WorldCat!