This oil thing is a red herring. If it was about oil, shouldn't we have invaded Venezuela instead?
IMHO Blair, at least, really did think that Iraq would turn into a stable democracy a bit like Turkey, Lebanon would follow suit, and the people of the middle east would see an alternative to corrupt dictatorships or theocratic revolution. Suddenly the breeding ground for Islamism would be no more, and just as happened in eastern Europe, liberal democratic capitalism would ensure peace and freedom for all.
Of course, this was magical thinking.
Like the Germans who still have a guilt complex over WW2, we still have one over our imperial past, which is why we are not keen on intervening in other countries. Something we need to get over.
Unfortunately, that's exactly the thought process that affected Blair and others with the concept of liberal intervention. Did NATO sending the Luftwaffe to bomb Belgrade not strike anyone as being in bad taste?