Secretary of Defense James Mattis embarked on his first official trip since being appointed head of the Pentagon on Thursday, landing in Seoul, South Korea to assure the nation of America’s commitment to its alliance.

South Korea, like many nations throughout the world, has developed concerns about its ties with the United States since Donald Trump won the presidential election last November. Trump suggested along the campaign trail that South Korea was not providing enough financial support for the thousands of U.S. troops stationed in their nation as a part of our commitment to defend them from their aggressive neighbor, North Korea.

Trump sent Mattis on this trip, which will include a visit to another wary Asian ally, Japan, to express America’s commitment to the standing defensive agreements the U.S. has in place with each of the two nations. These alliances have grown in importance in recent years as China has devoted significant resources to expanding its military presence in the region, particularly in the hotly contested South China Sea, which sees trillions of dollars in trade and contains significant oil and natural gas deposits.

“Right now we have to address the reality of the threat that your country and my country faces, and we intend to be shoulder-to-shoulder with you as we face this together,” Mattis said during his public discussion with South Korean Prime Minister and acting President Hwang Kyo-ahn.

North Korea’s continued efforts to develop nuclear-capable intercontinental ballistic missiles has made many world leaders uneasy, but none more so than South Korea’s acting president. North and South Korea have experienced a tenuous peace in the decades since the Korean War concluded, though it did not end formally, but ceased by way of a truce. Since then, the two nations have maintained a form of standoff at the border separating the two, positioning soldiers to stare one another down (quite literally) from across the demilitarized zone.

“South Korea and the United States must try to extract a change in North Korea’s strategic calculus by deterring the North’s aggression,” Hwang said in a statement to the press.

North Korean Supreme Leader Kim Jong Un claimed on New Year’s Day that his nation would be able to test its first intercontinental ballistic missile in the immediate future, though U.S. intelligence agencies are reluctant to confirm his claims. President Trump took to Twitter in January to make a clear statement regarding North Korea’s potential nuclear capabilities.

“North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the U.S. It won’t happen!” the president wrote on Twitter.