Internal affairs and international affairs intermingle with each other. Let's look at what is happening now: part of the world afire, and we not knowing which way to go and what action to take. More: divided, we don't know who are our friends and our enemies, and fear reactions inside and outside.
Powerful, we are impotent, with the USA aligning themselves on this regard with Europe. Once colonialist, Western Europe demised any claim to tell others how to be and what to do. Actually it even gave up telling it to itself.
Should I go over the history of colonialism, mainly in its Nineteenth Century version? It is easier to go over its end of the Twentieth Century version.
The newest version sprung from bad conscience over what went on previously. This raises the question: what did actually go wrong? Here the mainstream answer is summarized by the word imperialism (which embraces colonialism).
"Imperialist" are labeled people who want to impose themselves on others. Which now looks as a no-no, absolutely to be avoided. Hitler did it (actually also Stalin, but he is less talked about), but he was defeated, and we, the winners, don't want to hear of anybody wanting to implement a policy that runs against our grain. So anybody who trying to do it is likened to him (as it shows the mob of public servants parading in the streets of Madison).
On a larger scale, it is no longer a question of the Hitlers of the situation, but of "us": in the new era of blatant globalization, in which peoples seem to want to emulate us, or turn against us in hatred, "we" see ourselves as a kind of cancer of the earth, that bought about the ruin of all the pretty local realities everywhere existing before our arrival. It looks like "we" have imposed on others our life style – whose comforts, however, "we" are rarely ready to give up.
Hence, the heinous idea of multiculturalism: it looks open, for as much as it is closed: on "ourselves", incapable of understanding that what is culture for "us" it is simply the world for others.
Actually, colonialism is the history of mankind: all along a story of peoples' movements, their superimposing, mingling, or erasing each other. Never was there anything we could call "nativism". That local realities appear such is just another manifestation of the mystique of western domination.
Neither old days colonialists, nor to days multiculturalists, give sign of an idea of man, nature, society, and – last but not least – of God, such to convince and attract other peoples.
So the answer given to the question of what went wrong, turns out to be just another version of the same cultural disease, which led to colonialism and two world wars. The trouble, finally, is that arguing to convince others is confused with imposing, and any talk about truth becomes suspicious.