Is it true that, despite protecting us from all enemies foreign and domestic, that the Department of Defense cannot operate within the U.S.? If so, makes no sense to me, given oath. If not, why do we have a Department of Homeland Security?
Federal military personell may not participate in domestic law enforcement operations due to the Posse Comitatus Act of 1878.
Note that this act does not apply to non-federalized National Guard or state militia troops.
In addition to what friedo said (The US regular military can’t do domestic law enforcement, which is something I entirely agree with), I’d like to point out that the US military DOES operate a lot in the US. Since the vast majority of our enemies happen to not be located within our borders, the US military tends to do most of it’s active soldiering overseas. The few enemies that do manage to get within our borders tend to be under the jurisdiction of domestic law enforcement.
What does tend to happen within the US is a lot of training, logistics and administrative type stuff, and so forth. Texas is loaded with military bases where they do all sorts of stuff. Around here in College Station, it’s not at all that uncommon to look up and see Navy or Air Force jet trainers flying around (I’ve heard they use the airport nearby for touch-and-go practices sometimes). They have some huge bases where the Army and Air Force can go around practicing their thing. I’ve lived in a couple of places where I could sit on my back porch and listen to the Army doing life-fire artillery practice. In Lakewood, Washington, we were close enough to Fort Sill for the artillery to shake all the dishware in the cabinets. 
This was referenced in ‘The West Wing’, where a foreign diplomat (who was also believed to be a terrorist) was assassinated by US special forces.
I thought there would be a problem about diplomatic immunity, but the program concentrated on Posse Comitatus.
There is a claim that the Act also insists on the authority of Congress: '“SEC. 15. From and after the passage of this act it shall not be lawful to employ any part of the Army of the United States, as a posse comitatus, or otherwise, for the purpose of executing the laws, except in such cases and under such circumstances as such employment of said force may be expressly authorized by the Constitution or by act of Congress.”
http://westwing.bewarne.com/third/quibbles/law.html
But surely the OP meant actual combat, not training etc?
There’s another aspect to this. I was in the U.S. Navy “Seabees” for 23 years. There were a number of occasions where we could have performed, much needed, construction projects, both on military property and off, but we were prevented from doing so because it was seen as taking jobs away from the civilian sector. Often those projects never were done because they couldn’t be funded, even though we would continue to do construction just for training with no appreciable results. i.e.: erecting a steel building and then tearing it down, or moving earth around just to test the abilities of the planners and equipment operators. All the services perform functions such as this and training is necessary but, for the most part, can’t be used to benefit the gov’t. or civilian community.
I suspect you’re going to hear this same objection raised in relation to the construction being performed by Nat. Guard units on the U.S./Mexican border.
As I recall, the actual assassination took place off US soil, on an island somewhere where the plane made a fake emergency landing. They handled the diplomatic immunity aspect by not telling anyone they did it.
It may have been Bermuda (UK territory).
Err, not telling anyone doesn’t make it legal! :eek:
If so, then there is some serious confusion – the US military most certainly can and will fight ** on US home turf (or in territorial waters or over territorial airspace) if ** the defense “against all enemies” so requires and it is within their legal jurisdiction and operational preparedness. The red line is around Domestic Law Enforcement.
EVERY US Government oath of service includes the language* “defend the Constitution of the US against all enemies, foreign or domestic”. * That doesn’t mean the park guides at Yellowstone are authorized to deploy to Pakistan to chase Bin Laden.
The security policy of any sensible nation will seek to make it so that threats are dealt with by the optimally trained and organized entity. Armies deal with other armies or army-like entities, police agencies deal with common crime, specialized security outfits deal with espionage, sabotage and terrorism. For instance, the Air Force can most certainly launch missiles and fighter aircraft to intercept hostile aircraft over US airspace, and it can launch bombers and ICBMs from bases in the Upper Midwest to clobber some foreign location; however it’s not their job to send F-16s to drop 500-lb bombs on identified crackhouses in East Baltimore and we don’t want that to be their job.
The Posse Comitatus Act of 1878 was the device by which Republicans in Congress implemented their end of the “Compromise of 1877”. History books describe the Compromise as Democrats allowing Rutherford Hayes to win the disputed election of 1876 in return for a Republican promise to “withdraw federal troops from the South”, thus effectively ending Reconstruction.
Of course, this is nonsense. Federal troops were never withdrawn from the South; as Raguleader points out, the South is loaded with military bases even today, and Southern politicians have fought valiantly over the decades to maintain them. Rather, Republicans of 1878 enacted the Posse Comitatus Act, which provided that federal troops in the South couldn’t be used for federal law enforcement.
In 1878, there was only an embryonic Department of Justice, no FBI, and very few federal marshals and prosecutors. Taking the army out of law enforcement took the federal government out of law enforcement, allowing Southern state governments to all but eliminate African American voting and institute seven decades of apratheid-style segregation.
Over the ensuing decades, the PCA has become so riddled with exceptions and loopholes as to lose its significance. Eisenhower used the army to protect black students in Little Rock in 1957, Johnson used it to suppress riots in the 1960’s, and Reagan used it for drug enforcement. This article discusses some of these exceptions.
Nevertheless, the PCA remains on the books, and even though it was adopted for nefarious purposes, it does represent the (generally valid) principle that the army should stay out of domestic affairs in any democracy that doesn’t wish to become a banana republic. No doubt it will continue to frame debate over the appropriate use of the military in domestic crises.
The U.S. Military does conduct combat operations on U.S. soil. Remember Pearl Harbor.
Pearl Harbor didn’t become a state until 1957.
Hawaii wasn’t made a state until 1959, but it was still US soil since 1898 when McKinley signed the Newland Resolution. See Wikipedia.
Sorry, I wasn’t clear (and you have been!)
I meant domestic law enforcement, as opposed to training.
Commander Adama said it best on the new BattleStar Galactica series when talking to President Roslin:
Fort Sill is in Oklahoma, isn’t it? Did you mean Fort Lewis?
Which are the troops currently guarding Penn Station n NYC?
Those are New York National Guard troops under the command of the Governor (non-federalized).
They provide security but don’t do any actual law enforcement (although they could if the governor declared martial law.)
Another piece of law that folks have not mentioned: the Insurrection Act allows the President to use US troops – not just National Guard, but Active Duty troops – within the US during times of lawlessness, rebellion, or general disorder.
Very loud guns the Army uses in Oklahoma… Yah, I meant Fort Lewis. I used to live a short distance from the artillery range at Fort Sill as well though, and I remember sitting on my back porch listneing to their practices (and on a few occasions, watching what I assume was the Air Force dropping flares at night for some reason or another)
And of course, when I lived near Fort Hood in Texas for a while, there was one time when I got to see an Apache helicoptor fly over my house at a fairly low altitude