Mattis calls the Islamic State a "clear and present danger" and says he wonders why Congress hasn't acted already. He says an authorization from Congress to fight the extremist group would show resolve.
He was testifying today before the Senate Appropriations defense subcommittee.
Congress in 2001 gave President George W Bush approval to attack any countries or groups involved in the September 11 attacks.
The Trump administration, as did the Obama administration, relies on that nearly 16-year-old authority to send US troops into combat against the Islamic State.