Why would you think it would take the national guard, or any sort of serious force, or messing with routing tables?
All it would take two agents and a national security letter.
Most datacenters don't have that many people working in them at any given time. Well, there might be people, but staff is usually a half dozen or less security, 2 or 3 NOC techs, and some sales people if they even bothered to work from the office that day. Two agents say "cut power to all the gear in the meet-me room", and confirm everything went dark. Done. People argue less when confronted by people with guns, and put up less resistance when handcuffed and locked in a room. If they're feeling particularly ambitious (and horribly annoying IMHO), they could hit the main breakers on the equipment side of the battery room. No power to the routers, no data going over them.
Do a few major peerings, and the Internet is dead. A perfectly capable kill switch.
Something like this wouldn't be used for a SCADA problem. It's more of to isolate Internet resources.
In a war scenario, one of the first things an attacker would do is neutralize infrastructure. Power, communications, water, etc. Power is done at power plants. Only isolated pieces remain, like places with their own generators.
Communications is the point we're talking about here. Internet, land lines, cell phones, and sat phones down with the "kill switch". You'll still have some working, like PBXs within an organization, and some (but not all) sat phones. At very best, you'll have some sat phones talking to each other, but they aren't calling land lines or now disconnected cell phones.
Water is mostly killed with the power, or can be addressed later.
The only communications you'll end up with are HAM radios, and those with illegal transmitters switching to commercial AM/FM frequencies. Of those, it will only be the ones who have their own generators. They'll be a lot easier to triangulate without all the normal RF noise present.
Then you isolate consumable transportation. Civilians will starve without food supplies being brought in. Major roads, railroads, and sea ports with be blocked or severed.
The sad thing is, everything I described is in all of the war plans. It can also be accomplished by a small group of civilians with a good plan. It's simply proof that there has been no real threat to the United States. .. and to whoever may be reading this that says "Oh shit, he's a bad guy", I'm not. But if your bosses insist you interview me, feel free to stop by and say "hi". Bring soda and smokes, and we'll have a nice long chat. Don't mess up the front door, we'll open it for you.