"There is no choice but to have that discussion," Yuka Osaki said as he waited for a train at Tokyo station.

He professed no expertise in constitutional matters, Mr Osaki is an economics student at Tohoku University, but he was pleased to share a view on a topic that is generating angst in Japan.

Prime Minister Shinzo Abe wants to reinterpret Japan's pacifist constitution – or many suspect to change it, if necessary – and allow the country's military to fight alongside allies.

"It came about because of the US relationship," Mr Osaki said.

Not China, I asked him, knowing Japan's dispute with China over who owns islands in the East China Sea is never far from the headlines.

"That is not totally unrelated," he said, via a translator. "But is has more to do with Japan-US relations."

Japan has been an American ally since the end of World War II, when Washington imposed a pacifist constitution on the defeated nation. Japan's "peaceful orientation" in world affairs has become a national credo ever since.

"Japan must never wage a war again," Mr Abe declared a few months ago, but his words have failed to soothe a nation disturbed at plans for Japan to make what he describes as a "proactive contribution to peace".

He has established Japan's first National Security Council, made up of himself and three key ministers, to act in what officials describe as the "control tower" for Japanese foreign and defence policy.

The idea is simple, and was repeated often this week in a discussion with foreign ministry officials: Japan wants the right to do what other countries can do. To go beyond the right of self-defence allowed under the constitution to contribute to "collective defence", joining a battle should allies be threatened.

And in what must be seen as a historical irony, the move seems most welcome in Washington.

But the prospect of a more "proactive" Japan seems far from popular, especially at a time of growing tension with China over disputed islands.

Several thousand people turned out for a rally in downtown Tokyo on Wednesday condemning any hint of constitutional change. Beating drums and jostling against police, protesters held placards with a picture of Mr Abe adorned with a Hitler-like moustache.

Nobel laureate for literature Kenzaburo Oe reportedly warned the crowd: "I'm afraid that Japan's spirit is approaching the most dangerous stage over the past 100 years.''

Polls show the opposition is becoming more entrenched. Almost two-thirds of Japanese opposed the concept of collective defence, a poll taken by the Asahi Shimbun newspaper reported  – more than double the 29 per cent in support.

Similar opposition is to be found against revising the constitution.

But Mr Abe appears determined, and explained again this week the use of force would be kept to a minimum under collective defence. Officials present scenarios where Japan's military might act, such as in defence of US vessels on the high seas, intercepting a ballistic missile aimed at America, or using force to protect other countries' forces in peacekeeping operations.

These scenarios are helpfully illustrated with pictograms, with "Country X" always the anonymous label given to the aggressor. Officials insist no nation is being singled out. But curiously enough, in the pictograms the forces of "Country X" are always coloured red.

Daniel Flitton travelled to Japan courtesy of the Japanese government.