President Barack Obama was briefed Monday at the Pentagon and then went before the media to defend his strategy in battling the Islamic State extremist group while acknowledging it would take years to defeat the militants.

He also said that more work must be done to train and equip moderate Syrian rebels to fight Islamic State on their own.

The president also reiterated the administration’s position that in order for there to be success in Syria, Mr. Assad must go, a goal he said is gaining among the region’s major players.

White House officials cast the visit to the Pentagon as a routine update on the war against Islamic State. But the visit was far from routine, since the last time Mr. Obama appeared at the Pentagon was in October 2014.