Back in May, Prime Minster Shinzo Abe announced his plan to revise the famous Article 9 of Japan's Constitution which enshrines its unique post-war position as a "pacifist" state. Now, after this month's election, he is set to implement his plan. That means Japan is heading into its most searching debate for many decades about its place in the world and the foundations of its security.

Mr Abe won a landslide victory despite recent scandals and setbacks, thanks not only to a weak and divided opposition but also to the looming threat from North Korea, which naturally meant that national security questions loomed large for Japanese voters and played to Mr Abe's strengths in this area. That should help him drive forward his proposals to amend the Constitution to formally acknowledge the legality of Japan's formidable armed forces.

Even so, it will not be an easy task. Many Japanese, including many members of his own party, oppose any changes to Article 9. The heart of this opposition today is perhaps less a concern about the risk of a return to the militarism of the 1930s and early 1940s than an understandable reluctance to let go of the sense of security that Japan has enjoyed for so long under the protection of America. Japan's renunciation of war under the Constitution drafted by America is an integral part of the deal whereby America takes primary responsibility for Japan's defence. So preserving Article 9 is a way to hold on to American protection.

The problem of course is that, whatever is decided about Article 9, America's protection is becoming less and less assured anyway. America's promise to deter any North Korean attack on Japan will be much less credible when North Korean missiles can target nuclear warheads against US cities. And even more fundamentally, America's willingness to confront China if China threatens Japan is inevitably declining as China becomes more powerful. As the economic and strategic costs and risks of a conflict with China go up, America's reliability as an ally must go down.

And then there is Mr Donald Trump. Nothing he or his administration have said or done in office has reversed the impression he gave as a presidential candidate that he understood little and cared less for America's alliance commitments in Asia. His decision to cut short his forthcoming Asian tour and skip the East Asia Summit will only reinforce that impression.

TO BE A GREAT OR MIDDLE POWER?

So in a very real sense, Japan's debate about Article 9 over coming months will be a sideshow. The real debate will be about how Japan's strategic posture should adapt to an era of rising threats and diminishing allies. One option is to try to reinforce and reinvigorate America's strategic commitment to Japan by expanding Japan's ability both to defend itself and to serve and fight alongside America in confronting rivals like North Korea and China.

But Mr Abe has already taken the key step in that direction in 2014 when his Cabinet authorised "collective self-defence" operations so that Japanese forces could fight alongside allies even when Japan itself was not directly threatened. The reality is that more support from Japan - even direct military support from its very capable armed forces - will not do much to lessen the costs and risks to America of a war with either North Korea or China.

The reality therefore is that Japan must consider the kind of strategic posture it should adopt if it loses US support. That means especially how it would respond to China's obvious and growing ambition to become East Asia's dominant power. One possibility is that as America's strategic weight in Asia dwindles, Japan would step forward as the leader of a coalition to resist China's hegemonic plans and constrain its power and influence. That idea seems to lie behind Mr Abe's enthusiasm for closer strategic links with India, Australia and some countries in South-east Asia. It is a grand vision of Japan's re-emergence as a great power that appeals to those who, like Mr Abe, have never been entirely comfortable with the subordinate position that Japan has accepted since 1945.

But could it work? One obvious problem is other Asian countries' fears that Japan itself might still hanker after the kind of hegemony it tried to impose on Asia in the years before 1945. But those fears make no sense today when China's power so far outstrips Japan's. The bigger problem is that Japan might now be too weak to lead such a coalition. Other countries might not be confident that Japan would be strong enough to stand between them and China. They would also be unsure that Japan would be willing to do so, because its interests vis a vis China would not always align with theirs.

A second possibility for Japan is to consider a more modest posture. It would not seek to confront and resist China's ambitions in Asia directly. Instead it would accept China's emergence as the dominant regional power, but it would seek to maximise its own independence and freedom of manoeuvre in a Chinese-led order by ensuring that it could stand up to Chinese strategic pressure. This would be a classic "middle-power" posture.

And the third possibility is that Japan could simply accept China's predominance in Asia and try to make the best of it. One might call this the "Canada option", on the model of Canada's relationship with the United States. Of course this kind of posture would be much harder and riskier for Japan than for Canada, because Canada's relationship with America is so different and so much less difficult that Japan's with China. And many Japanese might well feel that this "small power" posture would be incompatible with Japan's identity.

But the reality is that this is the way Japan will be going as US power in Asia fades, unless it takes some big and very difficult decisions quite soon - especially concerning nuclear weapons.

Without the US nuclear umbrella, Japan will need nuclear forces of its own unless it is content to become a small power in China's East Asia. That is the real question that will lurk in the shadows as Mr Abe's plan to amend Article 9 unfolds.

•The writer is professor of strategic studies at the Australian National University in Canberra.