Correction: An earlier version of this column incorrectly reported the role of South Korea in Iraq and Afghanistan. This version has been updated.

B.J. Lee is a professor of international relations at Sookmyung Women’s University in Seoul.

On Sunday, South Korea signed a defense agreement with China. Under the accord, Seoul and Beijing will increase military hotlines and other exchanges.

Many Americans have overlooked the news. They shouldn’t. This is the most dramatic evidence yet that the Trump administration’s bullying of one of its most loyal allies is damaging fundamental U.S. interests in East Asia.

President Trump’s demand that South Korea boost its spending on U.S. troops in the country more than fivefold shows once again that he is willing to undermine a long-standing alliance relationship in return for domestic political gains. The White House is asking Seoul to pony up $5 billion for the 28,500 U.S. troops in the South in 2020 — up from $923 million this year. This might please his supporters at home, but it is seriously undermining the 70-year-old friendship between the two countries — a friendship sealed in blood.

South Korean troops fought and died alongside U.S. troops in Vietnam, and South Korea sent non-combat troops to Iraq and Afghanistan. Today, though, some South Koreans are speaking of American betrayal. On Tuesday, representatives from the two sides started what was supposed to be two days of talks on the costs of U.S. deployment — but the meeting ended abruptly after just 80 minutes. Seoul has already increased the amount it pays for U.S. troops by 8 percent this year. An almost 500 percent hike for next year is almost unthinkable.

Critics in Seoul say Washington’s behavior is approaching bullying, because it wants to set a precedent for upcoming negotiations for troop costs with Japan and NATO. Foreign Policy magazine recently reported that the United States wants Japan to raise its support for the 54,000 U.S. troops there by four times, to $8 billion.

Trump argues that the United States is paying too much for the defense of other countries, saying that wealthy allies such as South Korea and Japan should bear more of the burden. But he’s missing a vital point: The soldiers in Korea are there not only to protect the capitalist South from North Korea, but also to maintain U.S. military dominance in northeast Asia, where China’s rising military might threatens regional stability. Increasing military cooperation between China and Russia is another justification for the U.S. military presence on the Korean Peninsula.

The U.S. military presence overseas serves long-term U.S. interests by defending its allies’ democracies and free markets against authoritarian and totalitarian regimes. The current liberal international order built by the United States, and backed by its military presence overseas, has long helped the interests of U.S. institutions and businesses.

Nowhere is that truer than in South Korea. Thanks to help from the U.S.-led United Nations during the Korean War, the South not only fended off invasion from the North, but has now become a mature and prosperous democracy, an achievement the United States can proudly cite as evidence of its successful foreign policy. Facing North Korea, China and Russia to the north, South Korea is now the front line of the U.S. sphere of influence in the Asia-Pacific region. If South Korea is left to confront its far more powerful adversaries alone, the United States will lose a vital foothold on the Asian continent.

That helps to explain why the United States recently made a strategic choice to move its main military base from Seoul to the western port city of Pyeongtaek, one of South Korea’s closest points to China, just a few hundred miles across the Yellow Sea. The new base — the size of Washington, D.C., and the largest outside the United States — underlines the United States’ strategic imperatives. In addition to barracks and training fields, it has runways, eight playing fields, an 18-hole golf course and other amenities for 45,000 U.S. soldiers and their families. The garrison, opened last year, appears to be designed for a future when the threat of North Korea has vanished and if the two Koreas are unified.

What the Trump administration prefers to overlook is the fact that 90 percent of the $10.8 billion spent on the garrison’s construction was paid by South Korea. Seoul also points out that South Korea imported nearly $7 billion worth of U.S. weapons during the past decade, becoming the third-largest importer of U.S. weapons after Saudi Arabia and Australia.

So it should come as little surprise that a growing number of South Koreans are enraged by the Trump administration. Some even wonder whether the president’s outrageous demand is just an excuse for withdrawing U.S. troops from South Korea in case ongoing negotiations on the cost fall apart. That could be a worst-case scenario for both Seoul and Washington.

Read more: