Why was President Andrew Johnson impeached?

I have been doing some reading on the history of American presidents. I have done a lot of reading about Abraham Lincoln, and consequently have become interested in Andrew Johnson who took over the office after Lincoln was assassinated. My question is why was President Andrew Johnson impeached? I know that he often gets labeled as one of the worst presidents in our nation’s history, but I am not sure why he was impeached. I’ve read stories about how he was most likely drunk when he spoke at his inauguration for vice presidency and I’ve also read about how he handled the Reconstruction period very poorly. Despite these interesting anecdotes, I can’t find any in-depth readings that discuss the specific reasons for his impeachment. Can anyone help me?