I tried posting this in ELI5, but it was removed because politics aren't allowed. I read something about how he passed some kind of legislation that essentially preserved slavery in the South, but, admittedly, I didn't quite understand it. Basically, what laws/statutes did he violate that made filing for impeachment legitimate?
I didn't see this when I searched on Reddit, but I tried to search further on Google and found this from u/nixandnox from long ago:
For context: Andrew Johnson became president after Lincoln was killed. Johnson was only allowed into Congress after the war because he was the only Confederate senator to remain loyal to the union.
He was a Democrat back in the era where Democrats were very pro-white, pro-personal wealth, pro-acting like shit heads. (No seriously, they were awful.)
The reason he got to be Lincoln's Vice President was because the Republican party in 1864 needed a candidate who would appear loyal to Union-supporting Democrats.
After Lincoln's death, Johnson physically did everything in his power to oppose anything that tried make black people's lives better. He had been a slaver before the Civil War and he would have happily gone back, but the 13th amendment, which all Confederates States had to sign to be led back into the Union, freed all of the slaves. (The 14th actually defines citizenship.)
What actually got Johnson impeached (or at least brought up on charges) was Congress setting him up. There was a law in 1867 that said the president had to have the authority of the Senate in order to removal any government official who had been appointed with Senate approval called the Tenure of Office Act.
Johnson asked, they said no, and Johnson fired the guy anyway. After that, he had broken one of Congress's laws, so they had the right to Impeach him and they did.
For the rest of his term, he basically did nothing and didn't even make a puff about reelection.
Source: The American Promise, a concise version. ISBN:978-1-4576-3146-7