The first principle of Southeast Asian studies is the very artificiality of the concept of Southeast Asia. I have called this the “fundamental anxiety” of Southeast Asian studies, that there is no coherent argument why Southeast Asia properly includes Burma but not Bangladesh or Sri Lanka, or why it includes Indonesia but not Papua New Guinea, why Vietnam ought to fall within Southeast Asia rather than East Asia.
And yet Southeast Asia does exist. It is now a social fact. For example, Google knows exactly what Southeast Asia is and what it is is not.
Southeast Asian studies programs cover the same set of countries in the US, in Southeast Asia itself, in Japan, and elsewhere. The Association of Southeast Asian Nations will someday include Timor-Leste, and will never include Sri Lanka, Taiwan, or the Solomon Islands. Southeast Asianists have for decades lamented the intellectual strictures that Southeast Asian-ist thinking can place on their scholarly inquiry, and there is a rich genre of border-crossing Southeast Asian studies—on Zomia, most famously, but there are many others—that conceptualizes some subset of the peoples of the region as part of some competing configuration of peoples or states. Yet such inquiry has failed to dismantle the concept of Southeast Asia entirely, and my prediction is that it never will. Just look at how the Association for Asian Studies imagines Asia.
In my view, the durability of Southeast Asia as a social fact is easy to explain. The much more interesting question is, how did we get here? How did Southeast Asia become a social fact, and how did it become this particular social fact? Those eleven states, no more and no less?
The customary answer is that the concept of Southeast Asia is an external projection of Western images of the East onto the people living there. In some basic sense this is certainly true, but it leaves the details underspecified: why did the West happen to think that Vietnam is part of Southeast Asia, but not Bangladesh? Culturalist and geographic answers to such questions do not withstand close scrutiny; the Wallace line excludes the Philippines and half of Indonesia, for example, and Theravada Buddhist cultural influences are minor in much of Indonesia, the Philippines, and Vietnam. The answer upon which most scholars seem to settle is due to Ruth McVey, who looked to the idea of Southeast Asia as a lineage of the Second World War. This explains both timing and content of Southeast Asia, through
the coincidence between Southeast Asia’s birth as a concept and the triumph of American world power. World War II was important not for the bureaucratic detail of the South East Asia Command but for the fact that the Japanese occupied the region, creating an abrupt break with the era of European domination and making it an object of American attention.
McVey’s insightful analysis forms the core of my own understanding of why Southeast Asia is what it is. But the argument is incomplete because the South East Asia Command does not coincide with the social fact of Southeast Asia. Here are maps of Southeast Asia in the eyes of the Allied military command structure in 1942 and 1944.
Never were the Philippines and most of Indonesia part of the South East Asia Command. The South East Asia Command, in fact, operated out of Sri Lanka. And never was it the case that Japanese occupation demarcates the boundaries of contemporary Southeast Asia, even to a first approximation.
I am left without a good explanation for how Southeast Asia came to be this particular social fact. I wonder if readers can point me to other answers, no matter how grandiose or mundane they may be. It could be, for example, that Southeast Asia emerged as a social fact in response to the crystallization of other social facts, of South Asia, of China as including both the mainland and Taiwan, of Australia as a Western country, and of Melanesia and Micronesia as separate regions. Such an argument would raise other questions, but those questions too could help reveal why Southeast Asia is what it is.
tompepinsky.com 
Mallie Brossett February 11, 2016
I think it’s probably easier to generate reasons why certain countries aren’t a part of Southeast Asia that come up with one reason why Southeast Asia is defined the way it is. And most of it probably comes down to what you picture first when you think of that place. For example, that Bangladesh and Sri Lanka aren’t a part of Southeast Asia probably has to do with the fact that they were part of British India. So when people think of them, they think of them first as parts of the Indian subcontinent. When you think Papua New Guinea or the Solomon Islands you think about them in relation to Australia, or Taiwan in relation to China.
tompepinsky February 11, 2016
Thanks for reading. My own view is that you’re right, but that those observations still beg the question. So why do we think of PNG vis-a-vis Australia rather than Indonesia? And yes Sri Lanka and Bangladesh were part of British India, but so was Burma. https://en.wikipedia.org/wiki/British_rule_in_Burma#/media/File:IndiaPolitical1893ConstablesHandAtlas.jpg
Bernie Adeney-Risakotta February 11, 2016
These are good questions, which are quite puzzling. What bothers me more than the concept of Southeast Asia, is why is China, Japan, etc. called East Asia and not Northeast Asia? And why is India, etc. called South Asia and not Southwest Asia? If there is a fundamental anxiety in Southeast Asian studies (and I’m not sure if that is an appropriate way to characterize it), perhaps it is not about the boundaries of this “social fact” but that it’s giant neighbors (China and India) are designated by one geographical indicator (East and South), but our area is further fractionated (if that is a word). Perhaps one reason for the designation is that Southeast Asia has been influenced for millenia by the cultural influences of both China and India. So South East designates not so much a geographic space as a cultural and political reality, which has less to do with colonialism or American power, than it does with Sinisization and Indianization. India and China are two of the four great Axial civilizations (Bellah) and their creation of different kinds of mimetic, mythic and theoretic, modern civilizations have deeply influenced Southeast Asia,
Hadi February 13, 2016
A bit superficial and racist (but this is how the world sometimes is): maybe it is about just how the majority of people look? Anything east of Burma is brown, west PNG is (originally) black and you are left with a region that is south/not-China (+Taiwan) that is yellow.
Andrew Yong February 14, 2016
I think the origin of the idea of Southeast Asia is apparent from the older Western terms for the region – Indochina and the East Indies. Southeast Asia is the area in between the great civilisations of India (South Asia) and China (East Asia).
As to why certain areas are included and others not, the reasons are partly geographical/ethnographic and partly down to colonial history. Burma and Vietnam were influenced by India and China respectively, but they are culturally and linguistically distinct, and once Burma was separated from India in 1937, there is no impediment in terms of colonial boundaries to both of them being seen as Indochinese.
By contrast, Western New Guinea while geographically and ethnographically part of Australasia, is part of Southeast Asia because it was part of the Dutch East Indies and the Dutch failed to prevent it being amalgamated into Indonesia.
Ceylon has certain things in common with Southeast Asia as well as with India, but its geographic position on the other side of the Andaman Sea (with a land bridge to India in medieval times) make it difficult to group it in Southeast Asia.
Taiwan might have been Southeast Asian if it had not been settled and incorporated by the Chinese.
Shawn McHale February 14, 2016
Hi Tom. I don’t think the McVey argument really holds up, because it is too superficial. The creation of Southeast Asia as a social fact cannot be understood without understanding the creation, over centuries, of an understanding of “Indic” Asia and “Sinic” Asia. For a long time, Europeans tried to stuff what we think of as Southeast Asia into these catégories. “Southeast Asia” was often called “Farther India” — a concept that is centuries old, and covers most of what we think of as “Southeast Asia” today. (They may well have applied it as well to what we think of as southern Vietnam today.) But, given that the Indic legacy in *modern* Southeast Asia is quite circumscribed — e.g. found in royal rituals, spread of Ramayana, etc – calling this region “Farther India” did not make much sense in the modern era. (In 1945, Vietnam probably appeared the least “Sinic” than it ever had — so putting Vietnam in a Sinic category also seemed insufficient. )
I would argue that Southeast Asia, then, was born more as a *residual* category for a region that didn’t fit well into Indic and Sinic frameworks. It was composed of left-overs that did not fit elsewhere. Its borders were not all that clear, and its shared content not fully clear either. But one thing was blindingly clear: this area was no India or China. And, of course, once the category was created, scholars got to work in investing the category with a social reality. (Wolters later tried to do this, arguing, for example, that bilateral kinship and views about descent distinguished the region from South Asia and East Asia.)
Of course, politics intersected with this process. In World War II, you are correct that Southeast Asia Command did not include the Philippines — but this is largely because of a) egos — MacArthur did not want to serve under Mountbatten b) colonial fault-lines — how could one put a colonialist British general (Mountbatten) in control of the liberation of the Philippines, which itself had been slated to become independent in 1945? c) Naval versus Army — one also had to add to this mix the naval South West Pacific Area.
Shawn McHale February 14, 2016
Hi Tom. I don’t like the McVey argument, because it ignores centuries of attempts by Europeans and others to make sense of the region we now call “Southeast Asia.” The creation of Southeast Asia as a social fact cannot be understood without understanding the creation, over centuries, of an understanding of “Indic” Asia and “Sinic” Asia. For a long time, Europeans tried to stuff what we think of as Southeast Asia into these catégories. “Southeast Asia” was often called “Farther India” — a concept that is centuries old, and covers most of what we think of as “Southeast Asia” today. (It even covered about half of what we think of as Vietnam today.) But, given that the Indic legacy in *modern* Southeast Asia is quite circumscribed — e.g. found in royal rituals, spread of Ramayana, etc – calling this region “Farther India” did not make much sense in the modern era. (And of course, the only competition to this revised “Further India” model in 1945 would be the “little China” model, but this only applies to Vietnam and Chinese communities, so was insifficient for the region as a whole.)
I would argue that the Southeast Asia concept was essentially built on the “Farther India” one, but was born more as a *residual* category for a region that didn’t fit well into Indic and Sinic frameworks. It was composed of left-overs that did not fit elsewhere. Its borders were not all that clear, and its shared content not fully clear either. But one thing was blindingly clear: this area was not like modern India or China. And once the category was created, scholars got to work in investing the category with a social reality. (Wolters later tried to do this, arguing, for example, that bilateral kinship and views about descent distinguished the region from South Asia and East Asia.)
Of course, politics intersected with this process. In World War II, you are correct that Southeast Asia Command did not include the Philippines — but this is largely because of a) egos — MacArthur did not want to serve under Mountbatten b) colonial fault-lines — how could one put a colonialist British general (Mountbatten) in control of the liberation of the Philippines, which itself had been slated to become independent in 1945? c) Naval versus Army — one also had to add to this mix the naval South West Pacific Area.
Yue-En Chong February 14, 2016
Well, I would agree with the above commentators, I guess it would be looking at each state and asking what caused them to be ‘southeast Asian’ and which did not. But I think that South East Asia only makes sense from a much more complex point of view:
The Historical Reason for South east Asia
Bangladesh was part of ancient Indian dynasties so they would naturally be linked to India rather than to South East Asia. This is compared to Myanmar which lands were never taken over by any Indian dynasty for any considerable period of time.
Thailand was always shielded from any Chinese invasion because of Yunnan (which the Chinese Dynasties were having enough trouble keeping under their control)
How about Laos and Vietnam? While China had always tried to extend suzerainty over Vietnam, Imperial Chinese campaigns were never quite successful in establishing a presence in Vietnam.
By 1000, the area known as South East Asia actually looks kinda similar to the South East Asia that we know today – https://en.wikipedia.org/wiki/History_of_Laos#/media/File:Map-of-southeast-asia_1000_-_1100_CE.png
I’m not sure if anyone has ever done any research into this, but might there be a reason that Southeast Asia was intentionally left by China and India as a collection of small kingdoms and states to act as a buffer between the two great civilizations? A successful invasion of China of Vietnam, Laos and Cambodia and a successful expansion of India into Burma would mean that it was likely that both civilisations would have clashed across the rice fields of Thailand.
Of course, the above was unlikely as both the Indian and Chinese troops unused to warfare in malaria stricken tropical rain forest never managed to established a comfortable presence in Vietnam or Burma.
Geographical Reasons
Another useful map is this https://upload.wikimedia.org/wikipedia/commons/0/0c/East-Hem_800ad.jpg
It shows the mountain ranges that roughly cut South East Asia off from the physical occupation of either China or India. As mentioned above, the Chinese had enough problems with the Northern part of Vietnam and Yunnan to even consider taking over Southeast Asia.
Yet, geographical formations has historically served as natural boundaries to separate parts of the word from each other – one would wonder that without the Himalaya mountains and the mountains of Southern China if things would have been very different.
Trading Reasons
Instead, Southeast Asia was the neutral trade zone of the Indian, African, Chinese and Arab traders. One common feature of all South East Asian States is that they fought with each other over control of the entreport trade that went through the region (and they still do unto today)
This is how Indonesia becomes part of the larger south east asia narrative with them being just as much a rival kingdom as the mainland south east asian countries.
Ethnicity
As pointed out above, ethnicity also played a part in shaping the Southeast Asian identity. While there were large influx of Chinese and Indian immigrants into the region, it is undeniable that it was the Malayo-Polynesian language that gave a commonality to the people of Philippines, Malaysia, Indonesia and parts of Thailand and Cambodia.
Culture
While influenced by Indian and Chinese culture, the Southeast Asian culture maintained its own form having never been colonialised by either India or China for a significant period of time. This against gave it a distinctive culture of a place which was a blend of somethings Indian, somethings Chinese and then, Southeast Asian.
It’s probably due to a combination of the above factors that South East Asia is who they are today.
Political Reasons
Is there any reason why Philippines would want to be part of China? Or the Pacific? Or why should Myanmar want to be part of India?
We must remember that the concept of South East Asia, was consolidated with the signing of the Bangkok Declaration in 1967, prior to this it was British Malaya, Dutch Indonesia and French Indo-China. The creation of ASEAN helped to consolidate the idea of SEA even more.