When Chinese President Xi Jinping sits down with U.S. President Barack Obama tomorrow, he will be mulling a question that has puzzled China’s leaders and public for decades: Just how aggressive will a U.S. administration be in defending its Pacific allies from Chinese aggression?

It is hardly a theoretical question. Consider that last December, when China and Japan appeared closest to military conflict over territorial claims in the East China Sea, the influential Communist Party-owned Global Times newspaper offered a curious explanation for why China could not back down from the escalation: “The Chinese public will not allow such a retreat. A new reality has been formed surrounding the Diaoyu conflict. It is impossible to turn back.”

Only a few months earlier, perceived Japanese incursions on Chinese territorial claims spurred anti-Japanese riots with fierce, anti-government undertones. The message to the ruling Communist Party, just months away from turning power over to Xi, was clear: Pursue a more aggressive foreign policy or risk aggravating a public that is already fed up with corruption and incompetence.

Xi is now firmly in power, but the memory of those riots lingers, as do the disputes with not just Japan but also the Philippines and other U.S. allies. And while, for decades it was a foregone conclusion that a U.S. response to Chinese aggression would be stalwart and immediate, especially if China threatened Taiwan, things have changed. China’s foreign policy goals are even grander -- the country claims sovereignty over almost the entirety of the South China Sea -- yet its means of achieving them are seemingly more incremental (than, say, an invasion of Taiwan). The conflict with Japan concerns a set of uninhabited rocky islands notable for the (alleged) oil reserves beneath them. The conflict with the Philippines is over fishing grounds.

In both instances, Xi enjoys widespread public support -- from a hawkish military and the public -- to maintain diplomatic and military pressure to “recover” the territories in question. In the case of the Philippines and the disputed Scarborough Shoal (a large underwater reef and fishing ground), China has largely succeeded by permanently stationing ships in the area and daring the Philippines -- and anyone else -- to do something about it.

The Obama administration’s vaunted “pivot” to Asia, and its reallocation of diplomatic and military assets, is designed in part to meet China’s incremental but relentless drive to extend its sovereignty. In the short and medium terms, the pivot might succeed in slowing the expansion of Chinese power. But in the long term, it’s probably not lost on Xi or Obama that the political argument for extending Chinese sovereignty seems to resonate more strongly in China than the political argument for pushing back against China resonates in the U.S. As a result, the age-old question -- how far the U.S. is willing to go to back its Pacific allies -- appears to be slowly being answered in a way that favors China’s patient, long-term goals.

(Adam Minter is the Shanghai correspondent for the World View blog and a contributor to the Ticker. Follow him on Twitter.)