I took part in the annual ‘Doctoral Conference’ last Friday at my alma mater, the Fletcher School of Law and Diplomacy. After hazarding the Boston traffic, I presented my (and colleague Toshi Yoshihara’s) work on Taiwan’s naval strategy. The chairman of my panel, Prof. Sung-Yoon Lee, posed a fascinating question following the prepared remarks: which is the less trustworthy ally, the United States (vis-à-vis Taiwan), or China (vis-à-vis North Korea)? My answer: the United States.

Geography is a big part of the reason why. The Korean Peninsula is a half-island appended to the Asian mainland not far from the Chinese capital city, it shares a frontier with China, and it overshadows sea lanes connecting north China with the maritime ‘commons.’ Beijing can’t escape entanglement in Korean affairs.

This doesn’t mean China is automatically or permanently committed to North Korea. No doubt Chinese leaders would cut a deal tomorrow if they felt confident it would produce a unified, neutral Korea—even one governed from Seoul. (Prof. Lee pointed this out in a subsequent exchange of emails.) Such a grand bargain would lend stability to Northeast Asia, curing a perennial Chinese foreign policy headache. But at the same time, backing Pyongyang remains the default position for Beijing. Unification would spell the demise of the Kim regime. Thus, any process that yielded a peninsula governed by the South would place the regime on ‘death ground.’ Classical Chinese theorist Sun Tzu teaches that those who find themselves on death ground fight to the finish. Their survival is at stake, and they have nothing to lose.

The mercurial North Korean regime thus might lash out in some way or another if the leadership sees signs of Chinese abandonment. Pyongyang holds important Chinese interests hostage. Thinkers like Prof. Thomas Schelling point out that a reputation for being, ahem, less than fully rational confers negotiating leverage. That’s true between allies as well as adversaries. In effect, Pyongyang can threaten to destabilize the Sino-Korean frontier, perhaps by engulfing the Peninsula in war. Such a contingency could send refugees spilling into China in large numbers. It would certainly entail unforeseeable repercussions for Chinese policy. From Beijing’s vantage point, the status quo probably looks like the safer option when confronted with a North Korea whose foreign policy is premised on threats to run wild.

What about the US alliance with Taiwan? Here, too, strategic geography plays a crucial part, as does economics. If the Korean Peninsula faces the Chinese capital across the Bohai Sea, Taiwan is an island remote from the lives of ordinary Americans and the vast majority of their political leaders. It threatens to embroil the United States in war with its biggest trading partner during a time of economic malaise, and for little obvious reason. The US-Taiwan relationship is also complex. Rather than frankly vowing to defend the island against attack, Washington insists only that the island and the mainland settle their differences without resort to arms. It’s hard to found a constituency for protecting Taiwan on ‘strategic ambiguity’ of the sort long practiced by US leaders. Together, such factors discourage the United States from standing with Taiwan wholeheartedly as it confronts rising Chinese power. Washington’s refusal to sell Taipei advanced F-16 fighter jets is only the latest example of this.

It therefore behoves Taiwan to prepare to defend itself out of its own resources rather than banking on US support. And it behoves Washington to think carefully about the second- and third-order effects should Taiwan fall. Regional leaders probably see US-Taiwan relations as an outlier, and thus wouldn’t judge the credibility of US commitments to Japan, the Philippines, and other Asian states by its actions (or inaction) toward Taiwan. Even so, should the island return to mainland rule, and should the Chinese military position naval and air forces there, China will have turned Japan’s maritime flank—gaining near-complete control over the southern approaches to the Japanese archipelago. ‘As we obtain absolute security of our own maritime lifeline, it also implies absolute control over Japan’s maritime lifeline,’ proclaims Prof. Ni Lexiong, a leading Chinese sea-power proponent. The same would be true of South Korea, another important US ally in Northeast Asia.

Such a geostrategic realignment, then, would grant Beijing command of critical sea lanes, and new clout vis-à-vis longstanding US allies along with it. It would inject new unknowns into alliance relations and, conceivably, weaken transpacific ties as Tokyo and Seoul look to their own defences. I hope I’m wrong about all of this, and that the United States remains a dependable ally of Taiwan. More than the fate of one small island rests on it.

James Holmes is an associate professor at the US Naval War College. The views voiced here are his alone.