The privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion the public Safety may require it. (Constitution, Art. I, Sec. 9, Cl. 2).
Some people don't seem to understand what this means. I have read and heard more than one person suggest that because Al Qaeda attacked the World Trade Center in 2001, we are under invasion. This is not true. But even if it were true, it would be a moot point, because the public Safety would not require suspension of the Writ (to this point, the Government didn't even offer the invasion theory as an argument in favor of denying the writ to the detainees, rather they merely said the Constitution doesn't apply to those detained by our Government).
Let's see if we can break this down a bit, so you can follow along at home. First, we need to consider the argument - that we have been invaded. While one can easily point to the words of the President and those who support him on this war on "terror" and show that we are fighting them abroad so that we don't have to fight them here, let's delve a little deeper. Black's law dictionary (seventh edition, which is the most recent edition I have) defines invasion as: 1. A hostil or forcible encroachment on the rights of another (not applicable to our discussion), 2. The incursion of an army for conquest of plunder (definitely applicable to our discussion), 3. Trusts. A withdrawal from principal (metaphorically speaking, and also not pertinent to the discussion). It should be clear from the plain language of the definitions provided that the second definition is the one with which we should concern ourselves.
There are two prongs to this definition - the first includes the incursion of an army. 19 individuals hardly qualifies as an army, though they are tied to a network with a common goal, so perhaps, in the light most favorable to the apologists' position, one could construe Al Qaeda as an "army," though this is a stretch, because traditionally, armies have been tied to a country. The second prong, "for conquest or plunder" is the key to the definition. It is quite clear by the nature of the attacks (suicide missions) that the attackers did not intend conquest (Conquest, according to Black's Law Dictionary, is "An act of force by which, during a war, a belligerent occupies territory within an enemy country with the intention of extending its sovereignty over that territory. [emphasis mine]), and, just as clearly did not plunder (see "Pillage - the forcible seizure of another's property"), as they did not seize the property (Seize: "To forcibly take possession"). Looking at the legal definitions of the terms involved in an invasion, it should be glaringly apparent to everyone involved that the attacks on 9-11 did not serve as an "Invasion" of the United States.
Not that it would matter, even if it did. The Suspension Clause notes that the Privilege shall not be suspended "unless in times of rebellion or public invasion the public Safety may require it. (emphasis mine). Who determines what qualifies as a situation where the suspension is "required?" Well, anyone who has taken Constitutional Law should be able to tell you who the interpreters of the Constitution are, as denoted in Marbury v. Madison. The Court, of course, gets to decide what the Constitution means. And the Court has decided when the public safety may require the suspension of the Writ. In Ex parte Milligan, 71 U.S. 2 (1866), the Court addressed the concept of when the public safety may require suspension. In a nutshell, suspension is justified when there is such a breakdown in the region that the legal system can no longer function, thereby requiring martial law. The purpose of this is to limit the arbitrary use of executive power. It does not take a genius to see that there has been no breakdown of the legal system in Guantanamo Bay, short of the denial of basic legal protections for the detainees by the Government holding them, therefore, there is not backing for the premise that the Writ should be suspended.
This, of course, was known to the Government during oral arguments, which is why it was not brought up before the Court to decide, and the argument presented was merely that the Writ didn't apply. The majority of the Court disagreed, and (as I've said previously) rightly so.