People who dismiss ASEAN as an ineffectual "talking shop" make a big mistake.
But so do those who assume ASEAN will forever provide the answer to every strategic or diplomatic question in South-east Asia.
Like any institution, ASEAN's success has been a product of its circumstances and, as circumstances change, its future is not assured.
ASEAN's most important achievement can be easy to overlook because it has been so successful.
It has created and upheld the principle that its members do not use or threaten force against one another.
We have to remember how far this was from being true in the 1960s - and how far it is from being true in so many other parts of the world today - to see how remarkable that achievement really is.
The few occasions on which this principle has been violated only help to show how widely it has been respected.
Of course, ASEAN has done much more than that, particularly in promoting economic, social and political engagement among its members. But all of these achievements depend on ASEAN's primary legacy - peaceful interrelationships.
Keys to ASEAN's success
How did this happen? One reason of course is the statesmanship of a remarkable group of leaders among ASEAN's original members.
Typified by men like former Singapore prime minister Lee Kuan Yew and former Indonesian president Suharto, they had the vision to see how much their countries stood to gain by cooperation, and how much they had to lose from rivalry.
But that was not enough by itself. ASEAN's success also depended on a very favourable strategic environment in the wider Asia-Pacific region.
It is no coincidence that ASEAN was founded just as developments on the battlefield were about to weaken American public support for continued military action in Vietnam.
As Mr Lee has said, the United States' stand against communism in Vietnam gave ASEAN's first members the chance to consolidate their post-colonial political and economic systems, without which ASEAN could not have worked.
But more than that, the end of the Vietnam War also marked the end of the strategic rivalry between the US and China, which had fomented conflict in South-east Asia for two decades.
When then US President Richard Nixon met Chinese leader Mao Zedong in 1972, they ushered in a new regional, strategic and political order in which, for the first time, US primacy was uncontested by any major Asian power.
This provided the foundation for a remarkable era of peace and stability throughout East Asia because it banished the great power rivalries which had been the engine of so much regional conflict for centuries.
This has been vital to ASEAN's success because it has removed a potent source of discord between its members.
Strategic rivalry between great powers invariably leads them to compete for influence over smaller countries, as China and the US did in Southeast Asia during the Cold War.
ASEAN would not have flourished if China and the US had remained strategic rivals because they would have continued to try to divide ASEAN members against one another by drawing them into their competing spheres of influence.
End of US primacy
All this is very relevant to the question of ASEAN's future today. After 40 years, the era of uncontested US primacy appears to be coming to an end.
After 1972, China acquiesced to US leadership in Asia in return for recognition of China's communist government and access to Western finance, markets and technology to support China's growth.
Now, as China's economy seems poised to overtake the US', Beijing wants to revise the rules.
It seeks a bigger regional leadership role, at least equal to that of the US. It wants to be recognised as a great power again. The US' instinct is to resist China's challenge.
Unless the two nations can find a way to accommodate each other's conflicting aims, escalating rivalry is inevitable. Indeed, it is already happening in Asia today.
Washington v Beijing
There are also consequences for ASEAN. As rivalry grows between Washington and Beijing, each seeks to maximise its own influence among ASEAN members and minimise that of its rival.
Washington generally uses carrots, while Beijing has mostly used sticks but the aim is essentially the same.
Beijing's assertiveness over disputed maritime claims in the East China Sea has been intended, among other things, to show that the US cannot be relied on to defend the interests of its friends and allies against China.
The US' expressions of support for ASEAN members standing up to China have been intended to increase those countries' suspicions towards China and make them welcome a continued primary US role in regional affairs.
Both great powers aim to use the issue to strengthen their influence among ASEAN members. Japan's vigorous diplomatic efforts to win South-east Asian support in its disputes with China show a trend in the same direction.
The differences displayed at the ASEAN summit in Phnom Penh in 2012 show where this can lead.
ASEAN unity threatened
If these trends continue, there is a real risk that ASEAN's cohesion will come under further pressure.
This danger is all the greater now that ASEAN has members over a wider geographical spread. Geography makes a big difference to strategic relations.
The farther apart countries are, the less likely they are to see great power issues the same way.
For example, Vietnam will always see relations with China very differently from Indonesia, and it is simply unrealistic to expect that Indonesia will sacrifice its relations with China to meet Vietnam's interests, or vice versa.
As Asia's great powers become more strategically active, ASEAN's solidarity will be harder to sustain.
As new and sharper strategic questions arise in a more contested Asia, stalwart ASEAN members are already starting to look outside the organisation for answers.
Last year, Indonesia's Foreign Minister Marty Natalegawa raised interesting ideas for new mechanisms to manage escalating rivalry in which ASEAN had no central role.
He suggested that Asia's pressing strategic challenges would best be managed in a new forum encompassing countries from across Asia. This too is surely a sign of things to come.
This is not to suggest that ASEAN does not have an important role in South-east Asia's future.
It remains a uniquely successful and valuable organisation in many ways.
But for too long it has been too easy for South-east Asian diplomats to approach every problem saying "ASEAN will fix it". Today South-east Asia - and the whole region - faces new problems of a kind we have not seen since the organisation was founded. The region's policymakers therefore need to look for fresh solutions that may not involve ASEAN at all.
And the biggest and most urgent of these problems is simply this: What can the rest of us in Asia do to help slow the escalating rivalry in North-east Asia, which threatens the security of us all?
The writer is professor of strategic studies in the Strategic and Defence Studies Centre at the Australian National University in Canberra.
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