The general answer is that the U.S. government and its various agencies have indeed formulated several theoretical plans, as directed by higher-ups in response to the threat of major uprisings or other events, that would result in martial law and in the formation of centers that many would term "concentration camps". REX 84 is only one example. The ADEX list (of roughly 100,000 persons identified as "subversives" who were to be found and detained immediately in the event of the declaration of a nationwide state of emergency) and subsequent FBI databases (including computerized ones in known existence at this very moment) exist to catalogue and track people, both foreign and domestic, who would undoubtedly become "persons of interest" in a state of national emergency. The United States has already proven it can and will do this to sections of its population; specifically, Japanese-Americans detained en masse without cause or trial during WWII.
REX 84 specifies that the Constitution would be "suspended" upon the declaration of a "state of national domestic emergency" by the President. Martial law would be imposed, with members of the military taking over state and local government offices, and the rights and privileges contained within the Constitution would be unavailable.
Such a situation is unconstitutional in several ways; First and foremost, the Constitution of the Unites States of America is the supreme law of the land. It has no provision for suspending itself in its entirety, for any reason. The Framers weren't idiots; such a clause is ripe for abuse and would render the entire document worthless. The Constitution grants powers to the Federal government while reserving rights of the States and the People. Nobody in the Federal Government, not even the President or Congress, can supersede the Constitution or any of its provisions, except as those specific provisions may allow in specific situations. "Suspending" the Constitution in its entirety would be synonymous with repealing the Constitution, as any attempt to do so would be the end of the United States as we know it.
Next, FEMA is a bureaucratic arm of the Executive branch, in no way connected to the military. The power to command the Armed Forces rests several levels higher in the Executive branch with the President, and he may not delegate that power except as specifically prescribed by law; REX 84 is not law, and FEMA, in any case, cannot give orders to the military; it can only "coordinate" with military leadership, and their direction may be superceded by anyone higher in the chain than the officer commanding the troops in that area. As FEMA is currently under the Department of Homeland Security, which is 18th in line of succession to the Presidency, it is extremely unlikely that anyone who directly oversees FEMA would also have direct authority over the military.
Third, the Posse Comitatus Act requires that any military action taken to enforce "law and order" within the United States (basically using the Army as police) requires an act of Congress, just like a declaration of war does. The War Powers Resolution allows the President to send troops anywhere, for any reason, but only for up to 60 days unless further authorized by Congress with specific resolution approving the action, or a declaration of war.
Further, the Constitution doesn't only apply to the federal government. Ex parte Milligan states that while the suspension of habeas corpus (again requiring Congressional action) is legal and constitutional, the use of military tribunals to prosecute civilians of and in a State which affirms the Constitution and in which civilian courts can still function is illegal. Therefore, martial law (that is, the control of all areas of local government by the military) is illegal wherever state and local offices can function on their own (oh, and they haven't disavowed the Constitution and tried to secede). That's why Gitmo had to be created in the first place, and why attempts to move detainees to facilities within the U.S. failed or were aborted.
So, in order for REX 84 to become a long-term reality with any hint of legality, there can't be much left of law and order in the U.S.; the President must declare a state of national domestic emergency, Congress must OK the use of the military beyond 60 days, and must also specifically OK the suspension of habeas corpus. The President can then direct troops as he pleases, with standing orders to obey FEMA's direction. However, soldiers take an oath to obey the Constitution of the United States first, the President second, and their commanding officers third. In order of succession, if any of those say that what FEMA has said to do is wrong, the soldier MUST not do it.
THAT BEING SAID...
Yes, it is possible for the U.S. Government to start rounding up its citizens en masse. They don't need a state of emergency; they can do it now, without suspending the Constitution. The President has standing authority from Congress to detain any individual suspected of terrorism or of aiding and abetting terrorism, and to hold them in detention without trial or bail indefinitely. Congress can revoke this power, if it chooses, but if the President doesn't want to give it up, Congress must be able to override his veto (the last time that happened on something of this nature was the creation of the War Powers Resolution in the first place).
As long as the President has this authority, the FBI, which can commandeer local law enforcement for aid in responding to any terrorist threat within U.S. borders, can come to your home, take you from your spouse and children (who may also be detained), throw you in jail, and let you rot. All it takes is an executive order labelling you, or any group you claim membership of, as a terrorist, and there are several red flags which would apply to a large plurality of U.S. citizens:
- Owning a firearm
- Having more than one week's worth of food in storage (Sam's Club/Costco shoppers take note)
- Owning camping gear sufficient to survive for an extended period without electricity or running water
- Having said anything disparaging about the United States or its government in public (and the Internet is public)
- Having donated any money to a charity identified as being in support of a terrorist organization (If your church has ever donated money to any of these organizations, the entire national church congregation, including you, can be accused of aiding terrorism even if you personally never earmarked a penny of your offering)
- Having ever made a suspicious visit to a country in the Middle East (including Israel and Saudi Arabia; Christians visit Israel regularly as religious pilgrims, as do Jews, and it is one of the Pillars of the Muslim faith to undergo a pilgrimage to Mecca in Saudi Arabia)
