The music channel gave 40 young people from 24 countries the chance to ask him about his plans to bomb Saddam Hussein.
He told the audience: "It is nothing to do with oil, not for us, not for the UK, not for the United States".
But Blair had a hard job convincing the audience, most of whom seemed suspicious of the reasons for war.
He agreed war was "not the first answer" - he'd prefer Saddam Hussein to go into exile, rather than send British troops to war.
In the hot debate, Blair also said he was sure he could get the UN to agree on a second resolution on Iraq.
But he said if any of the UN nations vote against a second resolution without good enough reasons, the UK and US would probably go ahead with war anyway.
MTV Forum With Tony Blair: Is War The Answer? is hosted by DJ Trevor Nelson and will be shown on the MTV music channel globally on Friday 7 March.