WASHINGTON — Seeking war power authority for three years, President Barack Obama is setting up the prospect that his successor will become the third American president in a row overseeing U.S. military forces grappling with turmoil in the Middle East.

Passing on an active military mission in the region would be an unexpected and unwanted legacy for Obama, a president who once appeared poised to fulfill his campaign pledge to end U.S.-led conflicts in the region.

“It’s conceivable that the mission is completed earlier,” Obama said Wednesday as he urged Congress to pass a force authorization against the Islamic State group that would extend into 2018. “It’s conceivable that after deliberation, debate and evaluation, that there are additional tasks to be carried out in this area.”

“The people’s representatives, with a new president, should be able to have that discussion,” he added.

The president has long said the campaign against the Islamic State militants would be lengthy, suggesting it could extend beyond his presidency. But his authorization request to Congress makes that implication a reality and injects a complex foreign policy matter into the early stages of the 2016 presidential campaign.