That question the persistent mystery [read first] surrounding the (Pakistani) Indus Valley’s Harappan civilization and its big cities like Mohenjo-daro. It’s odd because their art, artifacts, and records seem to have no indication of military presence, even for defensive purposes. When you realize the society was a major contemporary of (and trading partner with) war-torn Mesopotamia and ancient Egypt, it suddenly seems almost impossible. And that’s without thinking about just how many wars there have been in the past 2,000 years.
I did some googling and quickly stopped because it turns out there’s a very elaborate “History” Channel-style or Discovery Channel-style conspiracy theory involving the topic, but I decided to offer some slightly more logical speculations, from the standpoint of trying to figure out the political and economic structure necessary to sustain such a long period of peace in the ancient world.
Although it would be harder to sustain peace over a long period in later times when populations were large enough to bump up against each other constantly — this would theoretically be possible/explainable under two easily imagined though not necessarily easily maintained conditions:
Condition 1. A very collective-thinking/communal culture over a large area could merge peacefully into a larger entity/civilization on the belief that more could be gained by merging than remaining independent (rather than merging by conquest as most classical and ancient empires did). One senses that with Harappan penchant for urban grids and brick standardization (while peer cities in other civilizations were virtually unplanned), they either had to be highly collective or highly authoritarian in character. And the latter basically requires troops that they didn’t have. (A third option would be somehow convincing everyone the leaders are gods on earth who must not be questioned or challenged, but that’s a hard facade to keep up for even a year.)
Condition 2. By following, as a foreign policy, the immortal principle “Don’t start none, won’t be none.” If you never attack your neighbor, it’s going to go a long way toward reducing the likelihood of a war with your neighbor because you’ve taken one of the potential triggers for an endless feud out of the picture.
That said, both of those conditions would seem to require a hell of a lot of restraint/forbearance and hoping — prisoner’s dilemma style — that your neighbor doesn’t suddenly flip on you without provocation. And given the history of early human species (not homo sapiens), it seems hard to believe you could get by for 2,000 years without your luck running out way earlier.
Although I suppose if you managed to find a little semi-isolated breadbasket (oh hey, maybe the river valley between the Khyber Pass and Thar Desert?) and develop enough surplus to both feed your excess regional labor and buy off your potential external attackers, you could really keep a good thing going for quite a while.
That policy, of course, really relies on not trying to hoard all the surplus at the top of the government/ruling class.