Bush made the remark in his weekly radio address on Saturday.
"…our troops will shift over time from leading operations to partnering with Iraqi forces, and eventually to overwatching those forces…"
Bush is following the recommendations of General David Petraeus, the top US commander in Iraq.
In a televised speech on Thursday, Bush announced he had approved Petraeus' plan to withdraw 5,700 troops from Iraq by the holidays and reduce the force from 20 combat brigades to 15 brigades by July next year.
On Friday, Defense Secretary Robert Gates raised the possibility of cutting US troop levels in Iraq to 100-thousand or so by the end of next year.