In his brief concurrence to the Court’s opinion today in Trump v. Hawaii, the decision Ilya discusses just below, Justice Kennedy adds an important point about limits on presidential power, even where the president has wide discretion, as here. Thus, he writes:

There are numerous instances in which the statements and actions of Government officials are not subject to judicial scrutiny or intervention. That does not mean those officials are free to disregard the Constitution and the rights it proclaims and protects.

And he adds:

Indeed, the very fact that an official may have broad discretion, discretion free from judicial scrutiny, makes it all the more imperative for him or her to adhere to the Constitution and to its meaning and its promise.

Put plainly, broad discretion does not afford the president access to any means toward the ends for which discretion is given. Kennedy illustrates the point with First Amendment religion and speech guarantees. He could also have invoked the Korematsu case the Court raised, as Ilya mentions. There the president enjoyed wide discretion to conduct the nation’s foreign affairs during World War II, but the means he employed in that case—incarcerating innocent Japanese-Americans—ran afoul of the Constitution’s due process guarantees, thus properly requiring judicial intervention in an area otherwise beyond the Court’s authority.