Cancer of the Colony
Colonial expansions leave three kinds of ex-/post-colonial nations in their wake: Rich & White, Poor, and Fucked. No matter which way you cut it, the indigenous people are never left in good shape. Here's how it breaks down.
The Rich & White: the colonists blast in, wipe out as many of the indigenous people as they need to to make room for themselves, and drop a whole new homeland-clone on their new patch. The colony becomes a new nation; any pre-existing social order has been obliterated along with the indigenous people. The few remaining (appropriately submissive) natives are allowed to take up a position on the lowest rung of the social ladder, where they'll be guiltily resented as a constant reminder of injustice.
Examples: New Zealand, Australia, the USA, Canada, Israel.
The Poor: the colonists drop in and place a new system of governance on top of the indigenous one, which is too mature, complex, or large to obliterate easily. The colony has a limited lifespan, either running out of steam or finding itself rejected by the host culture, but for its duration it leeches as much of the local labour and resources as it can, sending them back to the Imperial center. As a result, the progress of these countries is set back fifty or a hundred years, while the homeland uses the plundered resources to jump forward a similar amount. Eventually the colony is thrown off (or runs out of resources), leaving a shaken and retarded nation behind it.
Examples: India, the Pacific Islands, some of the Soviet Bloc
The Fucked: the colonists drop into a region of small-scale tribal or nomadic peoples and form a brand new nation-state. The new colony holds things together through a mixture of propaganda and violent coercion, making – for a time – a reasonable facsimile of its home culture.
Then things get a bit tricky, and the colony fails – sometimes volunteering to call it quits, sometimes being forced out at gunpoint. (Sometimes, perhaps, it never truly leaves, but ‘goes native.’)
In the wake of the colony are usually a couple of possibilities. The first is a complete breakdown. In Iraq, for instance, three very disparate groups of people discover it impossible to form a democratic government without a tyrant imposing order. Anarchy results. (Tempered only slightly by a second, undersized, military occupation.)
The other possibility is that locals decide to roll with the skeletal colonial structure left behind. Many of them. With conflicting ideas. The basic nation-state remains, but with conflict at every level as groups of indigenous people fight for possession, power, and survival. The previous equilibrium is disrupted and replaced with a new, toxic one.
Examples: Iraq, the whole African continent.
Of these three, the Poor are arguably in the best position. The Rich nations are fine, sure, but the remaining indigenous peoples are impoverished and powerless. The Fucked, well, life's no fun there for anyone except those at the very top of the pile – and their positions are never very secure.
There's no reversing the damage done. In most of these cases, it's not even going to be possible to fix it in any real sense of the word. And unfortunately, while the colonial aggressors have (mostly) reached the point where they realise the evil they've done and (mostly) should avoid too many future repeats (Iraq and Afghanistan obviously being salient counter-examples), the evil is propagating itself with no end in sight, due to the hundreds of millions of angry, injured people still suffering the effects.
Some people fall back on a defense of, for instance, apartheid in South Africa because ‘at least it was stable.’ I'd suggest that it was even more stable when there wasn't a South Africa at all. Similarly with Saddam Hussein's rule of Iraq. (1) Stability (relative to complete social breakdown) doesn't justify tyranny. (2) Complete social breakdown is not a consequence of the removal of the colonial order it sprung from; it's a consequence of the colonial order having been imposed in the first place.
Post-colonialism brings a whole new set of issues, from multi-nationals continuing to plunder resources to the original colonial powers continuing to interfere from afar in order to protect material interests. Post-colonialism is really just colonialism without the rhetoric of responsibility. This is probably a topic for a different post.