Last year, the United States asked South Korea for a hefty, 50 percent increase in financial support to cover some of the costs of basing 28,000 troops on South Korean territory. American and Korean negotiators eventually settled on a $925 million, one-year deal that marked an 8.2 percent increase from the previous year. President Trump, evidently dissatisfied by this sum, upped the ante this year and is demanding $5 billion, a 500 percent increase, in the current round of cost-sharing talks.

What makes Trump’s $5 billion shakedown especially vexing is the fact that South Korea has been a very good ally when it comes to burden sharing. Trump’s insistence that U.S. allies ought to bear a greater burden for their defense, a sentiment expressed by many previous U.S. administrations, is reasonable, but some allies—South Korea in particular—do a good job of shouldering their fair share.

It’s unclear how Trump came up with the $5 billion figure, but this seems to be a first push at getting an ally to pay his “cost plus 50” formula—the full cost of deployed troops plus 50 percent extra. Currently, South Korean payments go toward the salaries of their citizens employed as workers on U.S. bases and military construction expenses, though Seoul has not covered either of these two categories in full. According to the Pentagon, the $925 million Seoul paid in 2019 represents about 41 percent of the “day-to-day nonpersonnel-stationing costs” for American forces in South Korea. This percentage implies that South Korea would pay roughly $2 billion if it covered these costs in full, which comes to less than half of Trump’s demand.

South Korea is understandably flabbergasted by Trump’s $5 billion figure. Criticism in the South Korean press that the United States would be little more than a glorified mercenary force should Seoul cave to Trump’s demand is fair given that getting to $5 billion would most likely require South Korea to pay U.S. troops’ salaries. American negotiators walked out of initial talks, throwing into doubt hopes that the U.S. would eventually step back from the figure and give Seoul some breathing room. Additional reactions in South Korea have emphasized the significant erosion in trust between the allies as a result of Trump’s demand. Song Min-soon, a prominent and mainstream former diplomat, even raised the possibility of South Korea developing nuclear weapons to be less dependent on the United States.

South Korea has consistently demonstrated a serious commitment to burden sharing and self-defense in recent years. In July 2017, President Moon Jae-in announced that he would increase military spending from 2.4 to 2.9 percent of GDP during his time in office; most NATO countries don’t spend 2 percent of their GDP on defense. About one-third of South Korea’s military budget is devoted to the acquisition of highly capable, modern weapons systems including missile defense interceptors and radars, destroyers equipped with the Aegis battle management system, submarines, and the U.S.-made F‑35 aircraft. Seoul also picked up 90 percent of the $10.8 billion tab for the construction of Camp Humphreys, America’s largest overseas military base.

Trump’s cost-sharing hardball with South Korea will have knock-on effects for alliance politics in 2020. Similar agreements with Japan and NATO are coming up for renewal and negotiations on new terms will begin next year. According to a report by Foreign Policy, the Trump administration plans on asking Japan to increase their contribution from $2 billion per year to $8 billion, an increase of 300 percent. On the Korean peninsula, a rift in the U.S.-South Korea alliance created by the $5 billion demand will almost certainly hinder cooperation on diplomacy with North Korea.

Alliances should not be sacrosanct. The United States should not be afraid to demand more financial, military, and political support from allies that have much more at stake in facing regional challenges than far-away Washington. However, requesting a 500 percent increase in host nation support from South Korea—an ally that has consistently demonstrated its willingness to grow its monetary and military contribution to its own defense—isn’t a smart strategic move; it’s thinly veiled extortion.