Where will the US draw its defense line in East Asia?

TOKYO — One forthcoming U.S. security policy document has become the focus of intense scrutiny by major European and Asian governments.

U.S. allies are bracing for the anticipated release of the new National Defense Strategy — the central strategic blueprint that shapes U.S. defense policy both domestically and internationally — as early as October. Part of the regular four-year review cycle, the upcoming strategy could signal a significant shift in priorities.

The 2018 document, released during U.S. President Donald Trump’s first term, cast China and Russia as the primary challengers to the international order and pledged that the U.S. would prevail in strategic competition with them. That approach now appears to be in retreat.

By late August, the Pentagon had completed a draft of the new strategy, which is currently circulating within the U.S. government. According to news site Politico, the draft prioritizes defending the U.S. homeland over confronting major adversaries such as Beijing and Moscow, reversing the priorities set out in the 2018 strategy.

If true, such a shift could have far-reaching implications, as a more inward-looking military posture might strain U.S. security commitments in Europe and the Indo-Pacific.

« Since President Trump’s inauguration, the U.S. has prioritized homeland security, increasing border funding and deploying the National Guard and Marines domestically, » said Michael Shoebridge, a former Australian defense and security senior official and director of the think tank Strategic Analysis Australia. « The upcoming National Defense Strategy is expected to formalize these shifts. »

While some revisions may still be made before the strategy is finalized, the overall framework is unlikely to change. Since August, Trump has already been signaling a stronger emphasis on homeland defense.

That month, Trump deployed thousands of U.S. troops to the Caribbean, ostensibly to curb the flow of illegal drugs into the U.S. On Sept. 2, U.S. forces attacked a Venezuelan vessel suspected of carrying narcotics, killing 11 alleged members of a Venezuelan drug cartel in international waters. According to CNN, the administration is even weighing military strikes inside Venezuela to dismantle cartel networks.

By contrast, Trump has shown marked reluctance to mobilize U.S. forces in defense of allies. He recently informed the three Baltic NATO members bordering Russia — Latvia, Lithuania and Estonia — of his decision to cut funding for military projects supporting them, starting in fiscal year 2026. The move is a telling reflection of a homeland-first stance.

Naturally, few would question a sovereign state’s right to prioritize the defense of its own territory. The issue is how much U.S. engagement with allies will be sacrificed in the process.

In Europe, there is a growing sense of resignation that American military support for the region will decline. In contrast, some allies in Asia continue to hold out hope that, in the face of China’s rise, Washington will not only maintain but potentially expand its regional presence.

Sadly, such optimism may prove misplaced. The Trump administration does attach importance to Asia and is unlikely to cut military support there as sharply as in Europe. Yet if the White House doubles down on a homeland-first strategy, even Asia will not be spared.

Debates within the administration point to an unsettling reality: Officials appear to have yet to resolve the question of where to establish the defense line in Asia. This line represents the territorial threshold considered worth defending, even at significant cost, in pursuit of U.S. national interests.

In theory, there are four possible options for an Asian defense line the U.S. could draw. The most favorable outcome for regional stability would be a line encompassing Japan, South Korea and Taiwan, a posture Washington has broadly maintained for decades.

The worst scenario would be a strategic alignment that covers only Japan while excluding South Korea and Taiwan. Such a move could embolden North Korea and China, sharply increasing the risk of conflict.

History offers a stark cautionary tale. In 1950, U.S. Secretary of State Dean Acheson outlined a « defensive perimeter » in the Pacific that included Japan but conspicuously left out both South Korea and Taiwan. The so-called Acheson Line has long been blamed for giving Pyongyang the impression that Washington would not intervene, a perception critics argue helped trigger the Korean War that same year.

The two other possibilities are defense lines that exclude either South Korea or Taiwan. Even if Japan, host to numerous U.S. bases, remained within the perimeter, the omission of either South Korea or Taiwan would carry serious repercussions for Tokyo.

Multiple U.S. security experts said many uniformed officers, along with officials at the Defense and State Departments, support the first scenario. But within the administration’s intervention-skeptical camp, including Vice President JD Vance and others advocating disentanglement from foreign commitments, doubts persist over how deeply the U.S. should commit to defending South Korea or Taiwan.

At one point, a proposal quietly circulating within parts of the administration called for recognizing a South Korean nuclear deterrent in exchange for scaling back U.S. military involvement in the country’s defense against North Korea, according to a source familiar with the matter. The radical idea encountered pushback and was withdrawn, but it could still make a comeback.

On Taiwan, too, Washington is showing subtle differences over strategy, if not fundamental divisions. Hardliners in the military and in Congress, favoring a tough line on China, argue for a firm American commitment to defending the island, which Beijing insists is an inalienable part of Chinese territory. But some senior figures such as Vance demonstrate a more detached stance toward Taiwan’s defense.

Ultimately, the decision on where to draw the line rests with Trump himself. Given his past statements and actions, however, concerns regarding his judgment are difficult to dismiss.

Although he has railed against China over trade, Trump has shown little appetite for deepening U.S. military engagement in Asia to counter Beijing’s military buildup. He has shown no inclination to contest spheres of influence with China by viewing the world as a geopolitical chessboard.

A former senior U.S. official who knows Trump well said the U.S. president does not fully recognize that the U.S. and China have entered a new phase of all-out strategic competition for global leadership.

« U.S. allies must strengthen their own defense capabilities and reduce reliance on U.S. support, » Shoebridge said. « Investing in domestic defense industries and deepening cooperation with trusted partners, especially in the Indo-Pacific, such as between Australia, Japan, South Korea and the Philippines, is now essential. »

Even if Washington has retreated from its role as the « world’s policeman, » the fact remains that only the U.S. military has the capacity to slow the chain reaction of conflicts around the world. For America’s allies, the moment has come to coordinate more closely and apply united pressure to ensure Trump does not allow the defense line to slip backward.