How do you know it was the majority of the worlds population? There is no way to prove that.
For someone with a little time it is pretty easy to prove.
In the US, before the war started, it was greater than 50% against going without UN approval. See
this. Not surprisingly, this went to over 50% support once the Administration committed - I can't blame the US public for getting behind their President (although I wouldn't have got behind my own government, but then again I don't pledge alliegance to a flag either - we aren't as patriotic here. This isn't necessarily a bad thing).
In France it was about 80:20 (or higher) against. NZ was well over 50% against.
I'm guessing the Arab world was more than 50% against
. China and India - the real clinchers in terms of weight of population - I'd be surprised if they weren't over 50% against the war (ok, so I'm guessing now).
Why guess though. This suggests that India is against it - try
here.
OK, so that pretty much proves it. 87% times 900 million people = a world majority. There is no way you are coming back from that. I also found something else which suggested ~75% of Indians opposed the war from around the same time.
Please no-one bother picking apart the stats on this. I realise it is full of holes. The exact question matters - for instance, the US question in the CBS poll was "Should US invade Iraq without UN approval?". The demographics matter - the Indian poll only looked to the educated. But rather than focus in excruciating detail on these potential statistical errors, why not just accept that, prior to the war, the rest of the world was very much against a US-led, un-UN mandated invasion.
I also realise perceptions changed during and after the war (ie now). That doesn't make the invasion right.