I picked up Johnson two years ago during my annual time in Tennessee for the ACE quiz camp and subsequent extra days at Kingport's FunFest. With that said, I picked it back up again with the selfie in mind. Johnson was, for 130 years, the only impeached President in history. Then Clinton came along and messed things up so much they had to buy new signs changing that one fact about Johnson to stay updated.*
So why was he impeached in the first place? In a word: politics. It was Reconstruction, and with Lincoln gone, Johnson had the unfortunate fate of being the man who followed Abraham Lincoln (even if he was shot). Granted, Lincoln would have had the same problems with the Radical Republicans (and their grumpy head Tommy Lee Jones, I'm sorry, Thaddeus Stevens). The Radical Republicans passed a law call the Tenure of Office Act. Pretty much, the President couldn't fire anyone without the Senate's approval (sort of a reverse Advise and Consent). Johnson fired his Secretary of War Edwin Stanton and that was the trap. They impeached him on that and several other small matters, including giving three speeches insulting Congress to the American people. They got the impeachment but when it came for the Senate to remove him, it fell short one vote.
If you've never read JFK's "Profiles in Courage," do. There's a chapter devoted to Edmund Ross, a Senator from Kansas who was the one person to switch his vote and saved Johnson. As a result, he was voted out of office two years later.
*Also, let's get one thing clear. This is something I always always have to drill into my students' heads every single year:
NIXON WAS NEVER IMPEACHED!!!
You'd be shocked how many people still think he was. When I get around to selfiing Nixon, I'll elaborate.
Johnson is consistently ranked as one of the worst Presidents of all time. Sometimes he swaps spots with Buchanan every now and then (think about it: the two worst bookend one of the greatest). Does Johnson get a bad wrap? No. We was awful. Never mind he tried to carry Lincoln's banner; he just wasn't good at the job. When you never had a formal education that tends to happen. The biggest moral of Johnson's Presidency: stay in school.