The Japan-US relationship – like most nation-to-nation ties – is strongest when it’s unstressed and North Korea’s push for atomic weapons offers plenty of stress.

One result of this is talk in Tokyo on whether Japan should develop its own nuclear arsenal instead of relying on America’s.

It is also a reminder that Tokyo has long had doubts – even if never stated too loudly – about America’s commitment to defending Japan.

And in Japan questions are being asked about possible ‘de-coupling’ of the Japan-US defense relationship if Tokyo seeks an independent nuclear capability. But it’s also possible Japan might decide it doesn’t need the relationship at all.

Although the 65-year-old alliance is still sound enough, it’s not unbreakable – especially now that threats to Japan are no longer academic as they mostly were until a decade ago.

Read: American military suffering from myopia in Asia?

But what would it take to unravel the Japan-US alliance, with the first step being Japan building it’s own nukes?

It would take some significant incident in which America doesn’t do what Japan expects. What Japan expects is the Americans to protect Japan.

The Japan-US alliance is routinely described by both countries’ officials as ‘never been stronger.’ But the Japanese have had concerns for a while.

When the North Koreans first fired a ballistic Taepodong missile over Japan in 1998 and the Americans pressed Japan to not respond, a Japanese official commented: “Which one (North Korea or Japan) is your ally?”

And around 2010 when the Senkaku islands territorial dispute with China heated up, there was serious concern on the Japanese side that America would not get involved if China moved on the islands.

The US Government’s confused initial response over the Senkakus suggested the worries were not unfounded.

While President Obama later offered specific assurances relating to the dispute when visiting Tokyo in 2014 and US officials have confirmed this repeatedly ever since, doubts still linger. Especially as China steadily increases its civilian fishermen, coast guard, and military presence in Japanese waters near the Senkaku islands.

Meantime, North Korean nuclear tests and missile launches overflying Japan and even landing in Japanese waters are greatly worrying Tokyo.

Read: North Korea fires missile over Japan

There’s even talk of Japan developing a ‘strike’ capability to knock out North Korean missile launchers, along with the murmurings of developing nuclear weapons – something Japan can do without breaking much of a sweat.

One former official commented privately to the effect: Fifteen years ago it was a secret that we were even thinking about thinking about nuclear weapons. About seven years ago it was a secret we were talking about it. Now we don’t care if people know.

Former Defense Minister, Shigeru Ishiba who hopes to one day be Prime Minister, got attention recently for suggesting Japan consider a nuclear capability.

But as with many of Ishiba’s statements about defense, there’s an element of menace – suggesting that if the Americans don’t guarantee a nuclear strike when Japan snaps its fingers that Japan just might have to build it’s own weapons and then who knows what they might do.

Ishiba, like many Japanese politicians and officials, doesn’t understand Americans respond better to flattery than to being ordered around.

Regardless, the US government needs to clearly recognize Japanese expectations. A North Korean missile striking Tokyo will certainly get a violent US response. A Chinese invasion force landing in Kyushu? Same thing.

But what about a North Korean missile landing 50 miles offshore or hitting an unpopulated area in rural Japan? Or Chinese fishermen landing on the Senkakus and refusing to leave, while China’s Navy floats nearby warning Japan not to interfere?

Even in these borderline cases, Japan expects US support to the hilt and to include using force.

One can’t exactly predict when, where, or how such an incident will happen, but the US needs to thoroughly think this through, discuss it with the Japanese, and prepare itself psychologically to use force.

There will of course be a constituency in the US calling for a muted response – disguised as principled statesmanship – in order to avoid war with North Korea at any cost, or upsetting the economic relationship with Beijing over ‘some rocks’ in the East China Sea.

Fair enough, but if the US hesitates and doesn’t back Japan fully, then  be prepared to see the alliance unravel. This will also raise doubts among partners, potential partners, and enemies worldwide about all US alliances.

Of course, Japan can improve the odds of full US support by doing its part to spend a lot more on defense and improve cooperation between all US and Japanese military forces.

Read: US Marines and Okinawa – enough exasperation to go around

Suggesting that the Japan-US alliance might come apart is typically met with the sound of ‘harrumphing’ from academics, bureaucrats, and senior military officers on both sides.

But the Japan–US alliance is based on the idea the Americans will defend Japan.

Now that enemies have finally materialized, try backing off that commitment and we’ll see the glue binding the relationship together was soluble.