Many of the people who normally live in the city have already left, although thousands are still living there.
A curfew has started in Falluja, Baghdad Airport will be closed for 48 hours and people are being stopped from going over the borders with neighbouring countries Syria and Jordan.
A man called Abu Musab al-Zarqawi leads many of the people the US-led forces are fighting with.
His group doesn't want foreigners in Iraq, and has used violence and kidnappings to try and persuade non-Iraqis to leave.
One journalist in Iraq with US soldiers says they think the battle could be the biggest they've been involved in since 1968 in Vietnam.
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