They believed that the weapons were there, but thought Saddam Hussein had either destroyed them or moved them before the war started.
Whether the weapons were there or not is very important.
Mr Blair told the public that the UK was going to war because Iraq had weapons of mass destruction and was able to use them.
Now more and more people are wondering if Iraq did have those weapons, and if they didn't have them, why were British troops sent to Iraq?
Others say the war was okay because Saddam Hussein treated his people very badly, so invading Iraq to stop him doing that was okay.
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