Answer:
Serving as commander-in-chief: Under the Constitution, the president is the commander-in-chief of the Army and Navy of the United States, as well as of “the Militia of the several States, when called into the actual service of the United States” — in other words, the National Guard. The president does not have the power to declare war (that was left for Congress), but considering that the last formal declaration of war was in 1942, modern presidents certainly haven’t shied away from exercising their powers as commander-in-chief.
OR
Carrying out legislation: As the head of the executive branch of the federal government, the President is responsible for ensuring that all the nation’s laws are “faithfully executed.” In other words, the President carries out the legislation enacted by Congress but cannot initiate legislation himself. While constitutionally speaking the president is empowered only to sign or veto legislation that Congress sends to his desk, presidents have in recent years become more assertive in interpreting legislation through the use of signing statements. These statements often object to the provisions of a particular law on constitutional grounds and instruct executive branch officials how to implement the legislation according to the President’s interpretation.
Explanation:
The general thinking is that a law is unjust if it doesn't square with natural law. This is certainly the view that was put forth in Martin Luther King's Letter from Birmingham Jail.
So, a law that was passed that treated people differently wouldn't square with the natural law that all humans are equal. And thus, one would have the obligation to disobey that law.
