The visit comes a day after Iraqi Prime Minister Haider al-Abadi announced the start of the ground offensive on western Mosul, where Islamic State militants are essentially under siege, along with an estimated 650,000 civilians.
The insurgents were forced out of the eastern part of the city last month in the first phase of the offensive after 100 days of fighting.
The U.S. commander in Iraq, Army Lieutenant General Stephen Townsend, has said he believes U.S.-backed forces will recapture both of Islamic State's major strongholds - Mosul and the city of Raqqa in Syria - within the next six months.
Mattis said he aimed to get an up-to-date assessment of the war during his visit to Iraq. His strategy review could lead to additional deployment of U.S. forces, beyond the less than 6,000 American troops deployed to both Iraq and Syria today.
Experts say the Pentagon may also focus on other options, like increasing the number of attack helicopters and air strikes and bringing in more artillery.
The military may also seek more authority to make battlefield decisions, allowing commanders to take advantage of opportunities more quickly.
The future for U.S. forces in Iraq, and for Iraq's fragmented society, after the common enemy of Islamic State is expelled from Mosul is unclear.
Iran has close ties with the Shi'ite political elite ruling Iraq even as Washington provides critical military support to Iraqi forces battling Islamic State, a hardline Sunni group.
Mattis told the Senate last month that the top U.S. goal in Iraq should be "to ensure that it does not become a rump state of the regime in Tehran".
With the capture of Mosul in sight, a power struggle appears to be taking root between Iraq's Shi'ite leaders. Influential cleric Moqtada al-Sadr, who is openly hostile to Washington's policies in the Middle East, has begun mobilizing supporters ahead of parliamentary and provincial elections.
His main rival is former prime minister Nuri al-Maliki, a pro-Iranian politician re-emerging as a possible kingmaker or even for a return to the premiership itself.
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