COMMANDER-IN-CHIEF:President addressed US troops in South KoreaU.S. President Barack Obama on Sunday paid his first visit to the Demilitarized Zone (DMZ) around the tense border between South Korea and the Democratic People’s Republic of Korea (DPRK) aimed to highlight Washington’s security commitment to its ally as tension resurfaces. Obama, who arrived in Seoul earlier in the day to attend the two-day Nuclear Security Summit which opens on Monday, made a brief stop at the demilitarized zone before holding a series of bilateral talks with world leaders including his South Korean counterpart Lee Myung-bak. "You guys are at freedom’s frontier,” Obama told some 50 American troops gathered at the demilitarized zone, according to a press release. "The contrast between South Korea and North Korea (DPRK) could not be clearer, could not be starker.”The heavily fortified border area is symbol of lingering military tension on the Korean peninsula, where some 28,500 U.S. troops are based as a legacy of the 1950-53 Korean War. The trip came a day before the second anniversary of the 2010 sinking of South Korean Navy corvette Cheonan near the disputed western sea border between the two Koreas.A Seoul-led multinational investigation blamed Pyongyang for torpedoing the warship and killing 46 sailors aboard, but Pyongyang denied the charge.DPRK announced earlier this month its plan to launch a rocket- mounted satellite in mid-April in apparent defiance of a United Nations ban.    While the DPRK claimed the launch will abide by international norms and hit back its critics for continuing "hostile policy” towards Pyongyang, South Korea and the United States regarded the move as a test of a long-range missile. The Seoul summit is follow-up to the inaugural Washington summit in 2010 where Obama set the goal of securing the world’s nuclear materials by 2014.