He did it when he decided to fire the special counsel that was investigating him for Watergate. He didn't have the direct authority to fire him, so he ordered his Attorney General to do it, AG said no so Nixon fired him, then he ordered the Deputy AG, Deputy AG said no so Nixon fired him, next in line was the Solicitor General, Nixon ordered the SG to do it, which he did, if he didn't Nixon would probably go down the entire succession list up until a non-appointed official would come up who Nixon wouldn't have the power to fire.

https://en.wikipedia.org/wiki/Saturday_Night_Massacre