FORT GREELY, Alaska -- A small, remote Alaskan post bordered by mountains and moose herds, Fort Greely is America's major line of defense against long-range enemy rockets.

Inside its heavily-guarded Missile Defense Complex, rows of ground-based interceptor missiles that are tucked away in underground silos and radars dot the landscape. Within minutes, the missiles can blast off into space to collide with and destroy warheads speeding toward U.S. soil.

Using data from sea, land and space-based sensors, Soldiers of the 49th Missile Defense Battalion can spot a rocket launch from anywhere in the world. If an attack occurs, the 100th Missile Defense Brigade in Colorado Springs, Colorado, would then relay permission for the battalion to fire.

A number of recent rocket launches have originated from North Korea, which has threatened it could strike the U.S. with an intercontinental ballistic missile -- a powerful rocket capable of hitting a target with a nuclear weapon.

Despite the new threats, Lt. Col. Orlando Ortega, the battalion commander, said his well-trained Alaskan National Guard unit operates in a constant state of readiness.

"We've always been ready," he said. "Nothing really has changed from what we did a year ago. We were ready then and we're ready now."

Still, North Korea has kept the Soldiers busy.

Since late 2011, when Kim Jong-un took over after the death of his father, Kim Jong-il, the country has conducted four nuclear tests. The most recent was a miniaturized hydrogen bomb last month, which is believed to be the biggest test yet.

Earlier this year, Gen. Vincent Brooks, who leads all U.S. forces in South Korea, said Kim Jong-un has launched more than 120 missiles. That's twice as many missiles as his father and grandfather, Kim Il-sung, fired altogether in 40 years.

"Any launch that goes on, [Defense Department] sensors are able to pick it up and take it as a possible threat," Ortega said. "We will go through our procedures to ensure where it is going and if we have to defend the United States."

AT THE READY

The battalion has missile defense crew members who work 12-hour shifts around the clock so they can immediately react if a foreign country launches a rocket.

A five-person crew will staff a fire control node at all times. There, Soldiers have several computers, monitors and communication equipment to stay informed or relay any new developments. It's also where Soldiers can launch an interceptor missile to defeat an enemy rocket, if approved to do so.

By the end of the year, the U.S. military is expected to add eight more interceptors to its arsenal for a total of 44. The vast majority -- 40 -- will be based at Fort Greely and the rest at Vandenberg Air Force Base, California.

At Greely, crews train constantly to fight off complacency during their long shifts, in case they have to fire one or more of the missiles. Such training can include practicing real-life scenarios or learning complex software systems. Those skills are then tested by internal and external evaluations.

"If we're not at the top of our game, we are our harshest critics," said Maj. Bernard Smith, director of the Alpha crew. "We will do everything we can to get to the top of our game and maintain that level of proficiency at all times."

Team building is also paramount for Soldiers because of the consequences of failing to work as a crew. While confined together in a one-room node, the bond among crew members can grow strong.

With a diverse team of skilled Soldiers from Alaska to Puerto Rico, Smith said his crew is supportive and will often teach each other new things during shifts.

"I'm not very stressed when I come into work because I trust my crew -- we are a family," Smith, 50, said. "If there are any issues or problems they know we can come together as a crew, as a family, and accomplish anything."

ARCTIC LIVING

One of the major's crew members, Capt. Gilberto Ortiz, described the battalion -- made up of Soldiers on Active Guard Reserve status -- as a brotherhood.

About eight years ago, Ortiz left the warm climate of Puerto Rico for the arctic cold to protect the complex from intruders as a military police officer. When he heard of the missile defense crews, he decided to switch careers.

"It was something new [to me], something I didn't know about," said Ortiz, 32, who is now the crew's battle analyst. "I was like, 'OK, I'll do it. I'm up for the challenge.'"

Before coming on board, his first challenge when he arrived at Fort Greely was battling the weather, which can drop down to 60 below in the winter.

He learned how to wear layers, he said, and eventually acclimatized to the cold. He even earned the "Arctic" uniform tab after he graduated from the nearby Northern Warfare Training Center, which tests the cold-weather skills of Soldiers.

"The longer you stay here, the more you feel you are at home," said Ortiz, who lives with his wife and two children at Fort Greely. "[My] true home is thousands of miles away, but for me this is my second home."

When she landed her assignment, Sgt. Bethany Hendren was eager to leave Missouri and head up north.

"I fell in love with Alaska before I got here, because I love to travel and go to new places," Hendren, 29, said.

Similar to Ortiz, the sergeant was originally an MP and chose to change career fields while at Fort Greely. She now works as the crew's communications operator, where she monitors and reports all radio traffic on the ground-based fire control system.

While off-duty, she has another mission to help serve fellow Soldiers as the local president of the Better Opportunities for Single Soldiers program.

Being isolated in Alaska -- which only has a few hours of sunlight during the winter -- can sometimes impact a person's well-being. So, Herndon reaches out to Soldiers and invites them to events, where they can be more active and social.

"It is an austere environment and the winter months can be quite demanding at times," she said, "but I offer different programs and events for Soldiers to be involved in and to get out there and see Alaska."

'BULLET WITH A BULLET'

Fort Greely's infrastructure was made to be resilient, too. The post is self-sustaining with its own water supply and power plant, so operations won't be affected if electricity is cut to surrounding areas.

Security measures and sophisticated sensors can pick up any movement around the complex. Even when a moose or another animal triggers a sensor, MPs will always check it out.

"We take anything that comes in as a possible intrusion," Ortega said, "and we will have eyes on it physically and with cameras to ensure that it is not someone trying to come on."

All of these precautions go into being able to complete an incredibly difficult mission, where even the slightest miscalculation can result in failure.

Once an interceptor is launched, it releases a kill vehicle that travels roughly 15,000 mph in space and is maneuvered to collide with an enemy warhead.

"It is extremely hard to hit a bullet with a bullet," Ortega said.

The Missile Defense Agency tests interceptors out of Vandenberg AFB. While there have been past successful hits on targets in space, several interceptors can be fired at the same time, if necessary, to ensure an enemy warhead is shot down.

"We have confidence in the missiles and the capability," Ortega said, "but we will ensure with 100 percent confidence that it is destroyed."

That's a promise the entire battalion must keep, since over 320 million Americans depend on it.

As the lowest ranking member of her crew, Herndon -- just like the other Soldiers -- has embraced the huge responsibility of defeating a warhead if the time comes.

"We don't want L.A. or Seattle or any other part of our country to be attacked. It's as simple as that," she said.

"Of course we have our offense ready to go as well, but we're the only defense out there that can actually stop and prevent it from coming into our country," she added. "We're the shield."