US Defence Secretary James Mattis provoked a hostile response from China when he assured his Japanese counterpart Tomomi Inada on Saturday that the US alliance with Japan covered the disputed islets in the East China Sea known as Senkaku in Japan, and Diaoyu in China.

The rocky outcrops were transformed into a dangerous flashpoint when the previous Japanese government deliberately inflamed tensions with China by “nationalising” them. Prime Minister Shinzo Abe, who took office in 2012, further stoked the standoff by insisting the Senkakus were Japanese territory and ruling out any negotiations with China over the longstanding territorial dispute.

Mattis, who was visiting Japan and South Korea on his first overseas trip as defence secretary, was intent on reassuring both countries that their alliances with the US stand. During the US election campaign, Donald Trump repeatedly accused Japan and South Korea of not paying enough toward the upkeep of US bases and threatened to pull out of existing defence arrangements.

Mattis reiterated Washington’s support for the alliance with Japan in general, saying it was “critical to ensuring that this region remains safe and secure—not just now, but for years to come.” He specifically declared that the US-Japan Security Treaty applied to the Senkaku/Diaoyu islands—a commitment that President Obama gave in 2014 as tensions rose over the disputed islets.

Mattis’s remarks were precisely what Tokyo wanted to hear: a commitment by the Trump administration to wage war against China, a nuclear-armed power, in the event of a conflict over the Senkakus. It was the type of commitment that Trump derided throughout last year’s election campaign: the willingness of Washington to fight a war on behalf of other nations, in this case over tiny uninhabited islands of no immediate economic or strategic value to the United States.

Chinese Foreign Ministry spokesman Lu Kang reacted to Mattis’s comments by branding the US-Japan alliance as “a product of the Cold War, which should not impair China's territorial sovereignty and legitimate rights.” Lu urged “the US side to take a responsible attitude, stop making wrong remarks on the issue involving the Diaoyu islands’ sovereignty, and avoid … bringing instability to the regional situation.”

Beijing is deeply concerned over Trump’s threats to launch trade war measures and take an aggressive stance toward China over territorial disputes in the South China and East China Seas, as well as North Korea. Mattis visited South Korea before Japan and confirmed arrangements to install a sophisticated anti-ballistic missile system—Terminal High Altitude Area Defence (THAAD)—on the Korean Peninsula by the end of the year.

Beijing again protested against these plans for a THAAD battery, which is nominally aimed against North Korea, but is integral to the expanding US anti-ballistic missile systems deployed in Asia. This is part of Washington’s military build-up throughout the Asia-Pacific region for war with China.

Japan has already agreed to the installation of two high-power X-band radar stations that are critical to anti-missile systems. The Yomiuri Shimbun reported in December that the Japanese government was considering the purchase of a THAAD battery.

The US and Japan have jointly engaged in developing other anti-missile systems. It is no coincidence that on the same day that Mattis was in Japan, the two countries carried out a successful test near Hawaii of the latest version of the jointly-made SM-3 system, designed to shoot down short- and medium-range ballistic missiles.

Mattis further undercut Trump’s campaign rhetoric by praising Tokyo’s financial support for the more than 50,000 US troops in Japan as a “model of cost-sharing.” At their joint press conference, Japanese Defence Minister Inada declared that there had been no discussion over whether Japan should increase its funding for US bases.

At the same time, Mattis praised the Abe government’s growing military expenditure and suggested it be increased even further “in the face of the growing challenges we face.” He continued: “As our alliance grows, it will be important for both our nations to continue investing in our defense personnel and capabilities.”

The prime purpose of Mattis’s trip to North East Asia appears to have been to consolidate military ties with two important allies as the Trump administration prepares for confrontation with China. The very fact that the defence secretary chose to make his first overseas trip to Asia indicates that Washington’s main target is Beijing.

Mattis lashed out against China over its land reclamation and construction on its islets in the South China Sea, saying it “has shredded the trust of nations in the region, apparently trying to have a veto authority over the diplomatic and security and economic conditions of neighboring states.”

In reality, the Obama administration deliberately stirred up tensions over territorial disputes in the South China Sea in an effort to drive a wedge between China and its South East Asian neighbours. In his confirmation hearing, US Secretary of State Rex Tillerson went far further, warning China that the US would block access to Chinese islets in the South China Sea—an act of war.

Mattis sought to downplay any threat of immediate military action in the South China Sea. “What we have to do is exhaust all efforts, diplomatic efforts, to try to resolve this properly, maintaining open lines of communication. At this time, we do not see any need for dramatic military moves.”

While Japanese Defence Minister Inada publicly welcomed Mattis’s reassurances, the Japanese government undoubtedly remains concerned that Trump could renege on the guarantee over the Senkakus or ultimately walk away from the alliance with Japan completely. Foreign Minister Fumio Kishida hinted that Mattis’s remarks were not enough, saying Tokyo would seek to confirm the US stance on the islands “on various occasions.”

The erratic and bullying character of the Trump administration has sent chills down the spine of the Japanese ruling elite, like their counterparts around the world. The Financial Times reported that, in private, “senior Japanese officials have said one of their biggest fears is that Mr Trump could act unilaterally against North Korea, leaving them to face retaliation. One of their biggest early priorities has been to extract promises of consultation from the Trump administration.”

Prime Minister Abe, who was the first foreign leader to meet with Trump after his electoral win last year, is heading to Washington for talks with the new president on Friday in a bid to secure guarantees across a range of pressing economic and military issues.

The state of uncertainty surrounding Japan’s relations with the US is undoubtedly fueling a sharp debate in Japanese ruling circles over the need for Tokyo to more aggressively assert its own predatory interests.