By Kyu-seok Shim
SEOUL, Aug 18 (Reuters) - U.S. President Donald Trump's order to scale back joint military drills with South Korea has revived one of the most contentious issues from his first term, underscoring how his deal-focused approach continues to test the more than 70-year-old alliance.
Here's a look at how U.S.-South Korea ties have evolved under Trump.
HOW HAS TRUMP'S NORTH KOREA POLICY SHAPED THE ALLIANCE?
Trump moved from threatening North Korea with "fire and fury" in 2017 to meeting its
leader Kim Jong Un in Singapore in 2018, Hanoi in 2019 and at the Demilitarized Zone later that year.
After the Singapore summit, Trump suspended the August 2018 Ulchi Freedom Guardian joint exercise with South Korea, calling it expensive and provocative. Several subsequent drills were cancelled or redesigned.
Talks in Hanoi collapsed without a denuclearisation agreement, and North Korea subsequently expanded its missile and nuclear capabilities. The allies have since restored larger exercises, which now cover threats including drones and cyberattacks.
Trump's latest order revives his first-term approach under different conditions. North Korea has expanded its weapons arsenal, continues missile testing and has developed close military ties with Russia.
Nevertheless, Trump says his relationship with Kim remains good and described North Korea as "unthreatening and respectful" during his presidency.
HAS THE U.S. SECURITY COMMITMENT CHANGED?
Trump has repeatedly questioned the cost of maintaining roughly 28,500 U.S. troops in South Korea while insisting that Seoul remains an important ally.
His second administration has also pushed to "modernise" the alliance, potentially giving U.S. forces greater flexibility to respond to challenges beyond the Korean Peninsula, particularly China. That creates tensions for Seoul, which depends on Washington to deter North Korea but is reluctant to be drawn into a conflict over Taiwan, the democratically governed island Beijing claims as its own.
Negotiations with Trump have advanced some South Korean priorities, with the U.S. president last year endorsing Seoul's pursuit of nuclear-powered submarines.
Seoul also wants greater latitude to enrich uranium and reprocess spent nuclear fuel, as well as faster transfer of wartime operational control.
South Korean President Lee Jae Myung has boosted defence spending and pledged a larger security role, while maintaining that Washington's defence commitment remains firm.
WHY DOES BURDEN-SHARING REMAIN CONTENTIOUS?
During his first term, Trump demanded a sharp increase in South Korea's contribution to hosting U.S. troops, raising concern in Seoul that he might reduce forces if it refused.
The allies have a cost-sharing agreement covering 2026 to 2030, under which Seoul will contribute about 1.52 trillion won ($1.1 billion) this year toward the upkeep of U.S. troops.
Trump has nevertheless continued to argue that South Korea pays too little, while Seoul says the existing agreement should be honoured.
HOW HAVE TRADE TIES CHANGED?
Trump's first administration renegotiated a bilateral free trade agreement and imposed tariffs affecting South Korean steel.
His second term has brought broader tariff pressure. Washington initially threatened 25% tariffs before agreeing to a 15% rate in return for South Korean investment and other commitments.
Trump later threatened higher tariffs again, accusing Seoul of moving too slowly to implement the agreement. U.S. concerns have included South Korean regulation of American technology companies, with scrutiny of Seoul's treatment of U.S.-listed e-commerce company Coupang becoming a bilateral irritant.
WHY HAS INVESTMENT BECOME SO IMPORTANT?
South Korean companies have invested heavily in U.S. semiconductors, batteries, electric vehicles and shipbuilding.
Seoul also pledged $350 billion for U.S. projects in return for tariff relief. Disagreement emerged after Trump described the sum as payable upfront, while Seoul said it would involve loans, guarantees and equity and warned that a cash payment could destabilise its economy.
The detention of hundreds of South Korean workers during a 2025 immigration raid at a Hyundai-LG battery project in Georgia raised alarm in Seoul and prompted warnings companies could reconsider U.S. projects.
HOW HAS POLITICAL MANAGEMENT CHANGED?
Trump's two terms have largely coincided with liberal South Korean governments that favour engagement with North Korea.
Former South Korean president Moon Jae-in helped facilitate Trump’s diplomacy with Kim, but differences over sanctions and inter-Korean cooperation occasionally caused friction, including with Trump’s first ambassador, Harry Harris.
The conservative Yoon Suk Yeol government took a hard line on Pyongyang and strengthened security cooperation with Washington and Tokyo. Trump's second term has again largely overlapped with a liberal government under Lee, who also favours lowering tensions with North Korea.
Trump's personalised diplomacy has also prompted Seoul to cultivate unconventional channels into the White House. During the 2025 tariff negotiations, South Korean officials engaged directly with White House Chief of Staff Susie Wiles, whom they believed could clarify Trump's intentions.
The arrival of Seoul-born former Republican congresswoman Michelle Steel as ambassador gives Trump a politically aligned envoy as the two governments navigate differences over North Korea, defence costs, tariffs and investment.
(Reporting by Kyu-seok Shim; Editing by Ed Davies and Kate Mayberry)
