Was Andrew Johnson's impeachment justifiable according to U.S law?

Question

state your case and include a source if possible.

Answer 1

Not really. According to many historians it was simply a political ploy.

Answer 2

He broke the law. Breaking the law is grounds for impeachment.

He was acquitted, and the law was later found to be unconstitutional.

But at the time, it was the law, which he had sworn to faithfully uphold. He broke the federal law. That's grounds for impeachment (trial).