Much of what you say is quite true (re: the desecration/destruction of foreign sacred/cultural sites by colonial powers). But it's not just Western colonial powers - China and Japan wreacked havoc on the cultural sites and the artistic legacy of Korea for example, so much so that there are quite possibly more ancient Korean paintings in Japan now than there are in Korea itself! The wrecking and the looting of a country's cultural heritage often goes hand-in-hand with foreign or religious/political control. The British wrecked the Summer Palace just outside Beijing in 1860 and 1900. The Red Army, during the Cultural Revolution in China, wrecked its own cultural heritage. More recently the Taliban wrecked the Buddhist statues at Bamiyam (though thankfully it now seems probable that the gigantic task of restoring them will be undertaken http://www.thaipro.com/thailand_00/128_bamiyan_buddha.htm )

The above are all outrageous examples of desecration and vandalism inflicted by one belief system on another. The fact that the Elgin Marbles are at the British Museum however is a slightly different matter. While I agree with you in principle that they belong in Greece the fact of the matter is that even today, after decades, the Parthenon looks like a building site where the builders knocked off for tea twenty years ago and never came back! The corrosive air pollution of Athens is horrendous, and there is absolutely no doubt in my mind that had not the Marbles been removed, cleaned, conserved and kept in an environmentally controlled climate at the British Museum, they would not be in existence today - the Greek authorities should be mindful of that fact, and if they really do want to make a case for having them back they should get on with cleaning up their city and with the restoration of the Parthenon itself (the Elgin Marbles of course never will go back onto the building but into the new Acropolis Museum in Athens http://news.bbc.co.uk/1/hi/world/europe/7380279.stm - in other words from one museum to another).

As I say though I do agree with you in principle that artefacts and works of art should be returned to their country of origin if the country in question asks for them. It's not only the Elgin Marbles of course - the Dunhuang collection of paintings and manuscripts kept at the British Museum and the British Library is another candidate for return. I should point out however that the British Museum is legally bound not to disposed of, in any way shape or form, any item in its collection). Ironically, and on a personal note, I find it mind-boggling that most of the wonderful Anglo-Saxon Franks Casket, with is runic inscriptions, is in the British Museum while the right end panel of the Casket is in the Bargello Museum in Florence ;-)