He told other MPs that 850 British troops would be moving to a part of Iraq that is controlled by the US.
US army bosses want to move some of their troops from that area to even more dangerous places, and asked if some UK soldiers can fill in for them.
Mr Hoon said the decision to move the troops was based on advice from UK military bosses.
He added that the troops are from the Scottish Black Watch regiment and would be there for a matter of "weeks rather than months".
There are lots of armed Iraqis there who support a man called Abu Musab al-Zarqawi who wants foreigners out of Iraq.
The US wants to make Falluja a safe place and needs to move some of its soldiers there to try and take control of the city.
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