Obama visits troops at Korean DMZ ahead of nuclear security summit

President Obama made his first trip on Sunday to the tense Demilitarized Zone between North and South Korea, as tensions flared in the Korean peninsula.

Standing behind bullet-proof glass at an observation post, Obama got a quick-ten-minute look--peering through a pair of binoculars--at North Korea, which has threatened in recent days to launch a rocket next month.

Obama is the fourth president to visit the border, following Ronald Reagan, Bill ClintonWilliam (Bill) Jefferson ClintonMueller’s probe doesn't end with a bang, but with a whimper Mark Mellman: History’s judgment Congress should massively ramp up funding for the NIH MORE and George W. Bush.

During the visit--part of a three-day trip--Obama visited with U.S. troops based at a military camp near the 2.5 mile wide Demilitarized Zone in South Korea.

"You guys are at freedom's frontier," Obama said. "I could not be prouder of what you do."

Obama credited the troops stationed there for helping to make possible the "space and opportunity for freedom" on the peninsula.

"The contrast between South Korea and North Korea could not be clearer, could not be starker," Obama told the troops.

Obama is in South Korea to attend a two-day Nuclear Security Summit, where world leaders hope to announce new measures to prevent the proliferation of nuclear material.