Susan Walsh/AP Photo

PANMUNJON, Republic of Korea - Through a pair of binoculars and two inches of bullet-proof glass added especially for him, President Obama looked for the first time into North Korea today. In his first visit to the Demilitarized Zone that divides South from North, the president peered in person - if from a distance - at the nation threatening the stability of the region with its nuclear weapons program.

The president arrived in Seoul on Sunday for a three day Nuclear Security Summit, but his first official stop was at Observation Post Ouellette, where he took this personal look at the world's most unpredictable nuclear power: North Korea. The government of the rogue nation, now led by Kim Jong-un, warned that any criticism at the Summit of its nuclear weapons program would be considered a "declaration of war."

Mr. Obama's trip to Seoul for the biannual summit comes amid increased tensions after North Korea announced plans to test a long-range rocket next month, which the U.S. government believes is a ploy for testing long-range missile systems.

The president told roughly 50 of the more than 28,000 U.S. troops currently in South Korea that "the contrast between South Korea and North Korea could not be clearer, could not be starker, both in terms of freedom but also in terms of prosperity."

"You guys are at freedom's frontier," Obama told the U.S. troops, gathered at Camp Bonifas. "When you think about the transformation that has taken place in South Korea during my lifetime, it is directly attributable to this long line of soldiers, sailors, airmen, Marines, Coast Guardsmen who were willing to create the space and the opportunity for freedom and prosperity."

The president said that South Korean President Lee Myung-bak last year privately shared his personal story, growing up in brutal poverty, prospering as he grew up as did his country. Lee told him that "the only reason that was able to happen" was because of "all those American soldiers and the sacrifices that they made."

During his remarks to the troops, the president wore a dark windbreaker given to him by Gen. James Thurman, Commander of United Nations Command.

"I just want to point out that I was just presented this spiffy jacket," the president told the troops. "And so whoever arranged to make sure that it fit - I'm sure it wasn't the General - I appreciate it."

The president also met with a group of six South Korean troops at Observation Post Ouellette.

The president's trip to the DMZ came as North Korea marks 100 days since the death of its former dictator,"Dear Leader" Kim Jong-il. A siren from the north marking the end of the mourning period was heard as the president walked out to the bunker to look out over the barren landscape.

Visible from atop Observation Post Ouellette the president could see Gi Jong Dong to the North, known by the U.S. as "Propaganda Village," and Freedom Village to the South, the two villages permitted to remain in the 2 mile wide zone established as part of the 1953 armistice. The U.S. government says that Propaganda Village, which purports to house a collective farm, is uninhabited - a Potemkin village there for pure propaganda purposes.

-Jake Tapper and Mary Bruce