Much of the current legal wrangling goes back to a 1952 Supreme Court decision that found President Harry Truman didn’t have the authority to seize U.S. steel mills. In a concurring opinion, Justice Robert Jackson sought to enumerate the parameters of executive authority. Clearly, the president couldn’t defy an act of Congress but could use executive authority to follow legislative measures. In between, Jackson declared, there was a “zone” of concurrent legislative and executive powers that depended on the particular circumstances.

Nixon ultimately posed no problem because his acts violated the law. No president can order the Central Intelligence Agency to interfere with a domestic investigation or use the Internal Revenue Service to go after political enemies or order illegal break-ins. That’s the easy stuff.

It gets harder mainly on national security matters. Did Bush overstep in approving wiretaps without warrants or the use of torture after the Sept. 11 attacks? Likewise, did Obama go too far with drone strikes or using force in Libya without going to Congress?