President Barack Obama waves after delivering a speech at U.S. military base Yongsan Garrison in Seoul, South Korea, Saturday, April 26, 2014.

SEOUL - President Obama today hailed the U.S.-Korean military alliance as the "linchpin of security and stability" in Asia, crediting the sacrifices of American service members past and present with making it a reality.

"When our veterans come here and see this great and modern country, they can say with pride their effort and sacrifice was worth it," Obama said during a speech at Garrison Yongsan, a U.S. military base that's home to the Combined Forces Command (CFC).

"Freedom is not an accident," Obama continued. "These are things that have to be fought for … you're part of that legacy. It must be won and attended to constantly. And, here on freedom's frontier, they are … by every man and woman who has stood sentinel on this divided peninsula."

More than 1.5 million U.S. troops fought in the Korean War. Currently, more than 28,000 remain stationed here to help protect South Korea and provide training and humanitarian assistance. Some American military assets were also recently deployed in the waters off the coast to assist in search-and-rescue efforts after the Sewol ferry sunk April 16, killing more than 180 people.

"When our friends are in trouble, we help," Obama said. "Our hearts are broken for our Korean friends."

Obama also had tough words for North Korea - which he called a pariah state - warning that its nuclear pursuit will only lead to further isolation and asserting that the U.S. will counter any provocations by Kim Jong-un.

"We will not hesitate to use our military might to defend our allies, our friends and our way of life," he told the crowd of 1,500 troops and their families.

Ahead of the speech, Obama and South Korean President Park received a classified briefing on the security threat posed by North Korea from U.S. Forces Korea Gen. Curtis Scaparrotti.