WASHINGTON—President Barack Obama moved to expand the U.S. fight against Islamic State militants, authorizing up to 1,500 additional troops and asking Congress for $5.6 billion, while laying the groundwork for sweeping new legal authority to conduct counterterrorism operations across the Middle East and North Africa.

The White House plan, which would double the U.S. troop presence in Iraq, sets the stage for a broader foreign policy debate when Congress returns to Washington next week.

...