Outside the site of a new proposed US military base at Henoko on the southern Japanese island of Okinawa, three members of the American anti-war campaign group Veterans for Peace are singing songs while surrounded by a cordon of police to show solidarity with local people demonstrating against the facility. Tarak Kauff, Ken Mayers and Michael Hanes, who were deployed in Iraq and Afghanistan, are part of a delegation of six visiting the island, three of whom are former US Marines previously stationed on Okinawa.

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Local opposition to the presence of US forces on Okinawa, home to numerous American military bases, has been growing stronger again recently following a series of crimes by military personnel and base workers, including the alleged rape-murder of an Okinawan woman in April.

However, it is the threat from another American, presidential candidate Donald Trump, that has some in the Japanese government and military concerned about the future of the bases and the US-Japan Security Alliance.

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Trump has repeatedly attacked Japan during his campaign, criticising it over trade issues, questioning the validity of its alliance with the US and accusing Tokyo of not paying enough for the protection it receives.

“If Japan gets attacked, we have to immediately go to their aid … if we get attacked, Japan doesn’t have to help us. That’s a fair deal?” Trump asked supporters at a rally in Iowa.

Trump is running neck and neck with Hillary Clinton in many recent polls, despite his faltering debate performance, and even leading in some key battleground states, such as Ohio. Japan’s policymakers can no longer dismiss the possibility of a Trump presidency out of hand.

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Like South Korea, the potential ramifications for Japan are enormous at a time of growing threat from North Korea’s nuclear missile programme and an increasingly assertive China.

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On September 26, China flew a squadron of 40 military aircraft, including fighters and bombers, through airspace between Japan’s Miyako and Okinawa islands, resulting in the Japan Air Self-Defence Force scrambling fighter jets.

Newly appointed Japanese defence minister Tomomi Inada had days earlier said Tokyo would “increase its engagement in the South China Sea through … Maritime Self-Defence Force joint training cruises with the US navy”. The Chinese defence ministry warned that would amount to “playing with fire”.

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The annual cost of US bases in Japan is estimated to be around US$5 billion, of which Japan currently pays approximately US$2 billion. Throughout his campaign, Trump has called for it to make up the multi-billion dollar shortfall.

A senior Japanese defence official who spoke on condition of anonymity, said: “Because of the very serious budget situation of the Japanese government, it would be impossible to drastically increase the defence budget. US-Japan relations would face a time of turbulence. Many in the military and government are worried about the pronouncements from Trump.”

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In the light of a possible policy shift in the US, Tetsuo Kotani, senior fellow at the Japan Institute of International Affairs, said there was no option for Japan but to strengthen its own military. “Of course Japan must do more to increase its capabilities, particularly with missile defences. This US presidential election has stimulated that discussion. We should also consider offensive missile capabilities with the potential to strike the North Korean missile sites.”

The Japanese government proposed a record 5.17 trillion yen (US$51 billion) defence budget at the end of August. Despite being an increase of 2.3 per cent over last year, that budget would still only represent approximately a third of China’s military spending. The budget plan calls for a new submarine, enhanced anti-missile defence, new fighter planes and more troops on its southern islands. However, Japan still relies heavily on the protection of the US military, with around 54,000 troops stationed at 23 bases across the country, the majority of them in Okinawa.

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“The Obama administration had already undermined US credibility everywhere by stating that the US is no longer the world’s policeman. Even though US power is declining relatively, he didn’t have to say that,” said Kotani. “China, North Korea and Russia will now feel more confident in dealing with the US.”

Kotani sees Trump’s statements during the campaign as having further weakened US credibility abroad and believes the next administration, whoever it is led by, needs to recognise that fact and “will have to pay the price for it”.

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“From Japan’s perspective, we should prepare for all possibilities. One argument is that on the first day he becomes president, Trump will be briefed and shown classified documents that will make him understand the security situation,” said Kotani. “The other view is that he will never understand.”

But the fear is not just with Trump. Many policymakers in Japan see Trump as a symptom of changing US attitudes. “What I’m worried about is the shift in the American public as to what extent the country should engage international affairs. More and more Americans are becoming inward looking. This is a more serious cause for concern for the Asian region, not just Trump,” said the defence source. “Even Clinton has turned negative on the Trans-Pacific Partnership trade treaty.”

Gavin Blair has covered Japan for 15 years