Defense Secretary Jim Mattis, joined by Joint Chiefs Chairman Gen. Joseph Dunford, speaks at the Pentagon, on the US military response, along with France and Britain, in response to Syria’s chemical weapon attack.

:: US Defense Secretary Jim Mattis said on Thursday the United States would “probably regret” not keeping a holding force in Syria to ensure that ISIS militants did not re-emerge, the latest sign that a total US withdrawal was unlikely.

US President Donald Trump on Tuesday said he wants to withdraw American troops from Syria relatively soon but also appeared to temper that position by voicing a desire to leave a “strong and lasting footprint.”

A footprint, in military-speak, usually refers to a US troop presence.

Mattis did not offer any clear indication of how long US troops would remain in Syria or hint at future troop levels, stressing the ongoing mission to train local forces to combat ISIS.

“We have to create local forces that can keep the pressure on any attempt by ISIS to try to (re-emerge),” Mattis told a Senate Armed Services Committee hearing, using an acronym for ISIS.

When asked whether it would be risky to have local holding partners without US forces, Mattis said: “I am confident that we would probably regret it.”

Longer term US effort

The Pentagon and State Department have held that a longer term US effort will be needed to ensure a lasting defeat of ISIS. The group seized large swathes of Syria and Iraq but has gradually lost its territory since the United States and its allies started a military offensive in 2014.

Some of the harshest critics of a potential withdrawal from Syria come from Trump’s own Republican party, which blasted President Barack Obama when he withdrew US from Iraq in 2011. Iraqi forces began to unravel and eventually collapsed in the face of ISIS’s advance into the country in 2014.

“We learned that the Iraqi forces weren’t capable of providing security inside the country and that gave the enemy an opportunity to resurge, that is where really ISIS had the space to grow,” US Marine General Joseph Dunford, the chairman of the Joint Chiefs of Staff, told the hearing.

US airstrikes, troops and US-backed Syrian militias have dealt heavy blows to ISIS in Syria but the group still holds pockets of territory and is widely expected to revert to guerrilla tactics if the last remnants of its once self-styled “caliphate” are captured.

Mattis said he expected a “re-energized” effort against ISIS militants in eastern Syria in the coming days.

“You’ll see a re-energized effort against the middle Euphrates River Valley in the days ahead and against the rest of the geographic caliphate,” Mattis added.