Managing U.S. pressure on defense spending and the future of the Korea-U.S. alliance

Han Yong-sup

The author is the president of the Korea Association for National Strategy and the former vice president at the Korea National Defense University.

The second Donald Trump administration has identified China as the United States’ top military threat, followed by North Korea. Against this backdrop, Washington has urged Seoul to formulate joint responses to shared threats and increase its defense spending to 5 percent of its GDP. How the Lee Jae Myung administration responds to this demand has emerged as a major foreign and security policy issue.

Before addressing the request for a substantial defense budget hike, Seouth Korea must first approach the situation by dividing the alliance management framework into two categories: shared threat perception and the development of a joint defense strategy.

What are the military threats currently confronting both Seoul and Washington? The world has entered a new era of strategic instability. After the post–Cold War “age of diplomacy,” we are now in an age of renewed confrontation, marked by the wars in Ukraine, Gaza, and escalating tensions between Israel and Iran — including the recent U.S. airstrikes on Iranian nuclear facilities.

Since 2022, the United States has declared in its National Security Strategy that China, not Russia, poses the most significant geopolitical challenge and is the primary adversary seeking to reshape the global order by force. President Donald Trump has shifted responsibility for the war in Ukraine to NATO allies, focusing U.S. resources on strategic competition with China. During the recent NATO summit in the Netherlands, Trump succeeded in persuading all member states except Spain to commit to raising their defense spending to 5 percent of their GDP by 2035.

In contrast, South Korea’s perception of North Korea’s nuclear threat has varied significantly depending on the administration in power. Conservative governments have generally aligned with Washington’s concerns about China, while progressive governments have been more reluctant to endorse such views — a stance that continues today.

From the perspective of formulating a joint defense strategy, the United States aims to deter a potential Chinese invasion of Taiwan and to deny Beijing’s strategic objectives if conflict arises. To that end, Washington has developed an Indo-Pacific strategy that includes regional allies such as Japan, the Philippines, Australia and India. It also continues to expand the strategic flexibility of U.S. Forces Korea.

However, liberal South Korean governments have often prioritized dialogue with the North over deterrence and advocate for maintaining peace without confrontation. Such divergent threat assessments and response strategies strain the foundation of the alliance. Under Trump’s “America First” approach and transactional view of alliances, achieving a shared understanding of threats and strategies between Korean and U.S. leaders becomes essential.

Once that mutual understanding is in place, calculating the defense budget needed for joint operations and defining a clear division of responsibilities becomes a more rational next step.

Washington has long expressed frustration with Seoul's level of defense spending. While South Korea continues to request the sustained presence of 28,500 U.S. troops, regular rotations of nuclear-powered submarines and strategic bombers and ongoing combined exercises, it has reduced the size of its active-duty military from 600,000 to 500,000 and shortened conscription to 18 months for the Army and Marine Corps.

As South Korea has grown into one of the world’s top 10 economies, Trump and others in Washington argue that it can no longer afford to “free-ride” on U.S. security guarantees. In 2006, during North Korea’s first nuclear test, Korea’s defense spending was roughly 2.5 percent of the GDP, including cost-sharing for U.S. Forces Korea. Today, despite North Korea possessing around 100 nuclear warheads and directly threatening both South Korea and the United States, Seoul’s defense budget remains at a similar level. For Washington, this reflects Seoul's overreliance on U.S. extended deterrence.

If South Korea increases its defense spending, those funds could support the development of independent deterrence capabilities, including precision strike systems targeting North Korean strategic assets. Investment could also go toward building an advanced “kill chain,” AI-powered drone systems, acquiring U.S.-made Key Hole early warning satellites and establishing an integrated command-and-control system for nuclear and conventional warfare.

Rather than resist the Trump administration’s push for a 5 percent defense spending level, South Korea should use this moment to deepen joint threat analysis and formulate a shared defense strategy with the United States. That approach would strengthen the alliance and bolster Seoul’s status as a trusted global security partner in Washington’s evolving strategic calculus.

Translated from the JoongAng Ilbo using generative AI and edited by Korea JoongAng Daily staff.