Speaking at an Army garrison in a country where the U.S. keeps a presence of more than 28,000 troops, Obama said North Korea knows the path to prosperity and suggested its leaders take it.
"Because the Korean War ended where it began geographically, some used the phrase 'Die for a Tie' to describe the sacrifice of those who fought here," Obama said. "But as we look around at this thriving democracy and its grateful, hopeful citizens, one thing is clear: This was no tie. This was victory.
"This was a victory then, and it is a victory today," he said.
The president is in Seoul for meetings of the Group of 20 economic powers. He arrived hoping to seal an elusive trade deal with South Korea that could mean jobs and new markets for frustrated businesses and workers back home. Yet the deal was still in the balance in the last hours, slowed by U.S. demands over South Korea's auto trade and its market for U.S. beef.
Under worldwide pressure, Obama also told global leaders that they share the burden with the U.S. to fix trade-stifling imbalances and currency disputes that imperil economic recoveries everywhere. He promised the United States would do its part but declared "the world is looking to us to work together."
In the Veterans Day address, Obama said that, some 60 years after the war, the Korean peninsula provides the world's clearest contrast between a society that is open and one that is closed, between a dynamic, growing nation like South Korea and a North Korea "that would rather starve its people than change."
"It's a contrast that's so stark you can see it from space, as the brilliant lights of Seoul give way to utter darkness in the north," he said, describing the difference as a direct result of the road taken by the reclusive, communist north.
Obama said the U.S. "will never waver" in its commitment to South Korea's security and that North Korea's continued pursuit of nuclear weapons will only lead to more isolation and less security. He urged Pyongyang to take another path, a road that he said will offer its people growing opportunity instead of crushing poverty.
The commander in chief spoke inside a packed gymnasium, addressing a uniformed audience of service members from the different branches of the U.S. military. They surrounded him from all sides and many snapped photos as he spoke.
Obama condemned North Korea, saying its circumstances were not "an accident of history" but a direct result of the country choosing "a path of confrontation and provocation." That path, Obama said, includes its relentless pursuit of nuclear weapons and a deadly attack earlier this year that sunk a South Korean warship.
"In the wake of this aggression, Pyongyang should not be mistaken: the United States will never waver in our commitment to the security of the Republic of Korea. We will not waver," he said. "The alliance between our two nations has never been stronger, and along the with the rest of the world, we have made it clear that North Korea's continued pursuit of nuclear weapons will only lead to more isolation and less security."
Obama said North Korea has another path available to it.
"If they choose to fulfill their international obligations and commitments to the international community, they will have the chance to offer their people lives of growing opportunity instead of crushing poverty - a future of greater security and greater respect; a future that includes the prosperity and opportunity available to citizens on this end of the Korean peninsula," he said.
After the speech, Obama laid a wreath at a war memorial.
On the economic front, Obama was also to make his case directly to Chinese President Hu Jintao after lavishing attention on China's rising rival, India, for three days. The U.S. and China enjoy an economic partnership but continue to clash over currency, with the U.S. contending that China's undervalued yuan gives it an unfair edge in the flow of exports and imports.
The U.S. president made the point again in a letter to fellow leaders gathered here for the G-20 summit of established and emerging economies. Warning of unsustainable balance sheets, with some countries holding surpluses and others swimming in debt, Obama pushed for exchange rates based on the market and no more "undervaluing currencies for competitive purposes."
In less than two years on the job, Obama has become a familiar face at such summits, a sign of the enormous global effort to contain and reverse economic erosion.
He shows up this time on the defensive about the recent $600 billion intervention by the U.S. Federal Reserve, and weakened by a congressional midterm election that will give greater power to the opposition Republican Party.
Obama's message is that the United States cannot be the world's consumer, propping up others by borrowing and spending. He is pitching for a balanced recovery across the globe - tougher to achieve when national interests collide.
"The foundation for a strong and durable recovery will not materialize if American households stop saving and go back to spending based on borrowing," the president wrote.
Ahead of his trip, the Federal Reserve announced plans to buy $600 billion in long-term government bonds to try to drive down interest rates, spur lending and boost the U.S. economy. Some other nations complain that gives American goods an unfair advantage in competition with theirs.
Already pressed about that on his trip to Asia, Obama said the Federal Reserve acts independently, but still threw support behind the action. "I will say that the Fed's mandate, my mandate, is to grow our economy," Obama said. "And that's not just good for the United States, that's good for the world as a whole."
The president came into Seoul quietly on Wednesday night after his latest long flight across Asia, this one from Indonesia, where he had given a speech renewing his outreach to the Muslim world.
His upcoming agenda is packed with economic sessions and one-on-one meetings, in South Korea through Friday and this weekend in Japan. Obama's meeting with South Korean President Lee Myung-bak, after which both will take questions from the media, will surely center on trade. Negotiators ended a third day of talks on the stalled trade deal Wednesday, offering no clues on possible progress.
Within a few hours of Obama's appearance with Lee, the fate of the trade deal was far from certain.
"As we have said previously, if we can reach the standard for a fair trade agreement that the president has set out on, particularly autos, we will move forward," White House spokesman Jen Psaki said Thursday morning. "We hope to continue making progress."
At issue is a pact to slash tariffs and other barriers to trade, one that was signed in 2007 when previous administrations were in power. It remains unratified by lawmakers in both countries, and trade between the nations has slipped. The U.S. wants the deal to address an imbalance, and beef access to South Korea's market, before submitting it to Congress.
Obama will meet with Lee and German Chancellor Angela Merkel before the G-20 gatherings begin late in the day.
No longer in the emergency mode of preventing economic collapse, the leaders are seeking ways to sustain reliable growth and speed up the creation of jobs for their people.
Associated Press writers Kelly Olsen and Erica Werner in Seoul and Elaine Kurtenbach in Yokohama, Japan, contributed to this report.