The attacks of 9/11 greatly increased presidential discretion, which led to some infringements on individual rights and threats to the rule of law. One example is the U.S. government legalizing torture, which was considered unconstitutional before the attacks. Does the unitary executive allow the President of the United States to suspend the law at his discretion for purposes of national security? If not, what are the limits of the unitary executives and what can be done and not done in the name of national security according to U.S. laws?
The "unitary executive" is a theory that many, mostly conservative, legal scholars and advocates favor. It says that all executive agencies shall be fully responsible and responsive to the President, that the president may at any time order such agencies to follow any policy s/he directs. The Supreme Court has never adopted this theory in a decision. The Decision was rejected 7-1 in Morrison case with Justice Scalia the sole dissenter. But there has been a push to overturn Morrison.
In Seila Law v. CFPB, the court, while not adopting the theory totally, adopted enough of it to permit the President to fire the Director of the CPB at any time, despite language in the statute restricting this power.
This Vox article discusses the issue. It points out that the Seila Law case gave this power only over "independent" agencies headed by a single director. This is an unusual structure.
Nothing in the Seila Law case or in the "unitary executive" theory allows the President to generally override laws, even in the name of national security. It says only that certain laws limiting the power of the President over executive agencies are not constitutional.