The United States says it stands "poised to respond" at the border of North and South Korea, where U.S. troops are on high alert.

"They have ratcheted up their bellicose, dangerous rhetoric and some of the actions they've taken over the last few weeks present a real and clear danger," Secretary of Defense Chuck Hagel said.

The U.S. is sending a missile battery to Guam and two guided missile destroyers are on the border of South Korea, joining Navy warships already poised to shoot down an incoming missile.

U.S. warplanes, including fighter jets, U-2 spy planes and an A-10 attack jet, are in the South Korean skies today as part of a massive military exercise overseen by the U.S. lieutenant general who brought in F-22 stealth fighters as well. They'd be first into the North if war broke out.

(Ahn Young-joon/AP Photo)

"Their super cruise capability, their stealth capability gives them the opportunity to go places no other aircraft can go," Lt. Gen. Jan-Marc Jouas, deputy commander of the U.S. forces in South Korea, said.

In the secretive North stands the largest special operations force in the world. It is the head of the spear of the world's fourth largest military with 1,700 planes, 800 ships and 4,000 tanks.

But all the military and the nuclear weapons program, allowing them to threaten the world, has come at a very high price.

PHOTOS: Inside North Korea

At a border checkpoint, it is easy to tell where the North begins because there are no trees.

The people chop them down for firewood to keep warm and eat the softer roots. The lack of food and nutrition is so bad in North Korea that UNICEF estimates 28 percent of the children have stunted growth. One study found that on average North Koreans are a full two inches shorter than South Koreans.

Rare footage on Japanese television shows what are believed to be political prison camps where prisoners, men and women are kept on the verge of starvation, forced to do back-breaking labor.

With electricity scarce, North Korea is dark at night. It is a third world nation now commanding the attention of a superpower.

Despite North Korea's formidable military power, the U.S. is certain it would eventually be crushed in any type of war by the U.S. and South Korea but North Korea could no doubt inflict substantial damage if there was a surprise attack.

ABC News' Martha Raddatz and Brian Hartman contributed to this report.