Using the measures and regulations of one generation or one age to govern the world is like the case of a traveler in a boat who drops his sword in the middle of the river and notches the edge of the boat to mark the spot where the sword fell; then he goes back to the riverbank that evening to look for the sword below the notch on the boat. He is far from knowing what is what. – Huainanzi
In declaring martial law, the Constitution gives the President three kinds of discretion. The first is the discretion to determine what facts are relevant. The second is to determine whether, based on such facts, there is an “invasion or rebellion, and the public safety requires it”. The third is the discretion whether or not to actually declare martial law.
The first two kinds of discretion relate to whether the President can declare martial law, while the last relates to whether the President should declare martial law.
The Constitution provides for two checks to the exercise of these discretions.
The first is a legislative check wherein Congress, in joint session, can make a de novo or fresh or new review of all three discretions and make an independent determination not only whether the President can, but more importantly, whether the President should have declared martial law.
The second is a judicial check by the Supreme Court, but it can only review the first two discretions to determine whether the President can declare martial law.