Updated 8:42 a.m. Hours after being contacted by Danger Room, the New York Army National Guard on Tuesday night abruptly reversed a decision to send hundreds of soldiers out-of-state in the midst of the Hurricane Sandy relief effort.

The troops were previously declared unavailable to help New York recover from the state's biggest hurricane in centuries. Instead, they were assigned to fight a fake disaster.

But hours before they were set to deploy, the troops' participation in a week-long exercise in Missouri known as "Vigilant Guard" was cancelled. The exercise is designed to test the response to a mock earthquake in the Midwest. Until Tuesday, that previously scheduled drill took precedence over the real-world catastrophe that struck the East Coast. It was declared a bureaucratic near-impossibility to redeploy hundreds of guardsmen at a moment's notice, even at a moment when so many are in need.

Troops from the New York Army National Guard's 104th Military Police Battalion, the 1156th Engineer Company – 450 soldiers in all – were poised to head to the middle of the country. Dozens more from the 42nd Combat Aviation Brigade were supposed to train in Pennsylvania this weekend. "At this point in time, we're still sending our soldiers to Vigilant Guard," Eric Durr, a spokesman for the New York State Division of Military and Naval Affairs, told Danger Room late Tuesday afternoon.

Hours later, that changed. "Last night the decision was made to cancel New York National Guard participation in Vigilant Guard," Durr emailed on Wednesday morning. "The adjutant general [the Guard's commanding officer] decided to keep troops in state in case they are needed."

The New York Army National Guard has about 10,600 troops in total. Of those, about 2,100 are either in, or are waiting for, basic training. Another 400 to 500 are medically unavailable. And an additional 3,500 troops from the 27th Brigade Combat Team are either just back from Afghanistan or still there. Which means that New York's Army National Guard had, at most, 4,500 troops at its disposal – before the assignments to Vigilant Guard began. About 2,300 Army and Air National Guardsmen are currently deployed to the hurricane relief effort.

Hurricane Sandy has the most vicious storm system to hit the New York City area in nearly two centuries – a once-in-several-generations event that's left millions without power and tens of thousands of people homeless. The exercise in Missouri, however, is the second such drill in a year and a half.

New York Army National Guardsmen assigned to the 206th Military Police Company prepare to respond to Hurricane Sandy. Photo: Eric Durr, NYS Division of Military and Naval Affairs

The idea is to further prepare for a catastrophic earthquake along the New Madrid fault line, which lies on the border region of Illinois, Indiana, Missouri, Arkansas, Kentucky, Tennessee and Mississippi. The previous drill, National Level Exercise 11 – held last May and involving 9,000 National Guardsmen at 50 sites in seven states – was also conducted during an actual disaster. There were tornadoes assaulting the region at the same time.

Pentagon and FEMA leaders – especially Paul Stockton, the Defense Department's senior homeland security official – have been focused on New Madrid as the possible epicenter of a catastrophe to top all others, one that might leave millions displaced and rack up hundreds of billions of dollars in losses. "It’s so much bigger than anything we’ve faced — way beyond Hurricane Katrina," he told the Aspen Security Forum last year.

The New York guardsmen were scheduled to conduct mock operations at an urban search and rescue site in Columbia, Missouri, along with troops from Puerto Rico. "This exercise gives us an idea and appreciation for the process of moving our entire team and some of our equipment by air," Maj. Michael Tagliofierro, Homeland Response Planner for the New York National Guard, said in a statement.

It seemed like a good use of troops – until their fellow guardsmen were suddenly called in to help actual New Yorkers. In Howard Beach, Queens, soldiers from the 258th Field Artillery are patrolling the streets in order to keep a sense of order while the whole neighborhood is blacked out. "Between the burning fires, the turned over cars, and the people strolling around stunned," says Capt. Brian Napier, "it looked like a zombie apocalypse scene on Monday."

Other soldiers are continuing their rescue work, even though the chest-high waters have receded. A report comes in: 180 senior citizens are trapped on in a six-floor nursing home, and have to be evacuated. Many of them are too weak to walk, however, so the guardsmen have to carry them gently down the steps. It takes hours.

At the same time, the 106th Air Rescue Wing is operating out of Gabreski Field in eastern Long Island, assisting with the evacuation of the southern coast and using rescue boats to pick people out of the water. The New York Naval Militia has another several patrol boats on duty. Another 250 are in Manhattan, at the old armory on Lexington Ave and 25th Street. In Nassau County, nearly 600 troops are using military trucks and Humvees to help local law enforcement and to bring people to safety in remote locations like Fire Island. The guard already lost three Humvees to seawater that came up to the turrets."We're pulling people out of the attics of their houses," a source familiar with the operation says. "That's how high the water is."

Two hundred more engineers from Binghampton are on the way, as are 400 reconnaissance and surveillance specialists from Buffalo and Syracuse. Ordinarily, they fly and operate drones. But they're leaving the flying robots at home. This is old-fashioned rescue and recovery work, and it often requires lots and lots of human bodies to get it done.

How will the troops scheduled to deploy to Missouri help? It's not exactly clear. There are certainly occasions when the last thing that's needed in a crisis is more people getting in the way. Plus, these soldiers are ultimately at the service of the local civilian authorities. In Howard Beach, for example, Capt. Napier was taking orders from the local Fire Department chief. The civilians might decide they don't need to clog the streets with Humvees.

Still, if the troops participate in Vigilant Guard, the civilians don't get to make that choice. Those soldiers are taken off the table for the response to this disaster.

Flooding on East 6th Street and Avenue C in Manhattan on Monday night. Photo: Flickr/david_shankbone Flooding on East 6th Street and Avenue C in Manhattan on Monday night. Photo: Flickr/david_shankbone

All told, the U.S. has now activated more than 7,500 National Guard troops for service in the affected states, with the bulk of them concentrated in New York and New Jersey. Troops are working with New Jersey state police for search-and-rescue operations, and 750 troops are helping clear debris in Virginia. Humvees, medium tactical trucks and 150 guardsmen were also deployed to Washington D.C.

The Army Corps of Engineers have sent emergency power teams to help repair infrastructure, and the Nevada National Guard is airlifting a rescue boat and para-rescue specialists from California to North Carolina. A Coast Guard helicopter crew managed to save 14 sailors from the HMS Bounty after the famous replica ship sank in heavy seas on Sunday.

U.S. Northern Command, which oversees military action on the continent, has adapted several bases to work as "Incident Support Bases," including Westover Air Reserve Base in Massachusetts; Dover Air Force Base in Delaware; and Joint Base McGuire-Dix-Lakehurst in New Jersey, which has 250 ambulances available for disaster operations. The command has also positioned eight search-and-rescue helicopters at Hanscom Air Force Base in Massachusetts. FEMA is staging out of the Army's Fort Devens base in Massachusetts. The Pentagon has also appointed six brigadier generals from the Guard to act as "Dual Status Commanders" in charge of both state and federalized troops.

That includes Brig. Gen. Michael Sweezey of the New York Army National Guard, who has now changed his mind, and decided he better have as many troops as possible on hand to help in Sandy's wake.

– with Robert Beckhusen