The plaintiff in Saleh was an Iraqi immigrant who, along with other members of a class who sought to bring claims against high officials of the Bush Administration for what they suffered during and after the American invasion. He claimed that Donald Rumsfeld and others had planned an illegal war and used the panic that ensued after the 9/11 attacks as an excuse to put the plan in motion. (There is ample evidence for this, but one need only point to the Bush Administration's consistent insinuation that Saddam Hussein sponsored the attacks.)
Despite the defendants admissions, and the reports of multiple official inquiries conducted into the illegality of the Iraq War that concluded that the United States had committed the "supreme international crime" of waging aggressive war, the case was dismissed. The plaintiffs were not even permitted to present the shocking evidence against Rumsfeld et al. that they had compiled. The courts concluded that former government officials were (or would be) immune from suits alleging that they had committed war crimes.
American courts have acknowledged that there are certain offences that are so serious that that those who commit them are the common enemies of mankind, who can be prosecuted in any court. The Nuremberg defence that a war criminal was "only following orders" and doing what was not illegal in a foreign country has been categorically rejected. However, the judiciary has not allowed suits against American war criminals to proceed; Saleh was dismissed because these former high officials were immune from prosecution in American courts. It is not the nature of the crimes that mattered, but the status of the defendants.
No American has been prosecuted for violating the most fundamental set of rights while conducting the "war on terror". Those who tortured detainees were immunized by act of Congress, while those who planned that torture (including Donald Rumsfeld, again) were protected with the doctrine of qualified immunity or Westfall Act (which require the implicit conclusion that planning war crimes was part of their official duties),while the whistleblowers who revealed these crimes were locked up.
As I detail in my book, wherever a possibility remained to bring actions against US officials, the courts went out of their way to tell the legislature how to take action to eliminate it. In Saleh, the opinion concludes--gratuitously--that Congress could immunize executive branch officials from all claims for violation of jus cogens norms (including torture, slavery, and genocide). (If this seems far-fetched, one should remember that in 2003 Congress passed the American Service-Members Protection Act, which authorizes the President of the United States to invade the Netherlands if any American is put on trial in the Hague for war crimes.)
The Saleh opinion was released on a Friday, so that it might pass through the news cycle unnoticed, while the (ultimately insignificant) denial of the stay in Washington v. Trump was released to great fanfare. The latter waxed poetic about the rule of law to no significant effect, while the former landed a muted but devastating blow. How can there be a rule of law if there is no forum where the executive can be held accountable?
The short answer is that without courts willing to hold the executive responsible for violations of the most fundamental rights, there is no rule of law (this is my book's main argument, by the way). This really should surprise anyone familiar with the history of our constitutional tradition, which emerged as a response to the argument that no one had the right to hold the executive accountable. The world's leading human rights lawyer (Geoffrey Robertson) detailed this history in The Tyrannicide Brief. As every schoolchild used to know, King Charles I, when put on trial, spoke only once, at the outset of the proceedings. He began his infamous speech by challenging the court's power to judge him:
"I would know by what power I am called hither ... I would know by what authority, I mean lawful."
The only answer to this question is the rule of law. If governments are permitted to re-empower the executive to plead in the same manner as Charles I and the defendants at Nuremberg, we have abandoned the idea of a government of laws, and not of men. One might as well allow the president to wear a crown, or let his assistants carry fasces.