The main reason the US and UK went to war was because they believed Saddam was hiding weapons of mass destruction.
The weapons haven't turned up yet. But Mr Blair and US President George W Bush say they're still confident they will be found.
Mr Blair also said the UK would continue to stand by the US and praised the President.
But he said the US should listen to other countries to get their support.
The American politicians were bowled over by Mr Blair's speech.
They gave him 17 standing ovations - that's when people stand up to clap because they're so impressed.