President Obama is prepared to use U.S. military airstrikes in Syria as part of an expanded campaign to defeat the Islamic State and does not believe he needs formal congressional approval to take that action, according to people who have spoken with the president in recent days.

Obama discussed his plans at a dinner with a bipartisan group of foreign policy experts this week at the White House and made clear his belief that he has the authority to attack the militant Islamist group on both sides of the Iraq-Syria border to protect U.S national security, multiple people who participated in the discussion said. The move to attack in Syria would represent a remarkable escalation in strategy for Obama, who has sought during his presidency to reduce the U.S. military engagement in the Middle East.

Administration officials have been working in recent days to enlist the support of the nation’s political establishment to help sell their strategy to the American public, which Obama will address in a prime-time speech Wednesday night. The president met with the top four congressional leaders Tuesday, while his aides held briefings on Capitol Hill. […]