WASHINGTON — For a president criticized as overly cautious and reluctant to lead, Barack Obama is taking a huge risk. He is thrusting U.S. fighting forces into a growing military operation with clear dangers, unknown costs, an indefinite length and unpredictable consequences.

After years of resistance, the president who wanted to end America’s wars will now oversee a sweeping airstrike campaign in both Iraq and Syria, a country mired in an intractable civil war. He’s sending hundreds more U.S. troops to Iraq to help train security forces there. And he’s pressing Congress for authority to pour U.S. weaponry into Syria to strengthen opposition forces fighting both the Islamic State militants and President Bashar Assad’s government.

All three are precisely the scenarios Obama has assiduously sought to avoid.

For now, the public is with him, with polls showing wide support for airstrikes in Iraq and Syria even as Obama’s own approval ratings slump and his foreign policy ratings sink to near record lows.

“This is America’s leadership at its best,” Obama declared in his address to the nation, as if to answer critics even in his own party who complain he has been too slow to act.

But as determined as Obama sounded Wednesday night, his resolve could be sorely tested by the uncertainties of war and the threat of Americans being killed or captured. “Any time we take military action,” Obama said, “there are risks involved, especially to the servicemen and women who carry out these missions.”

White House critics have argued for years that Obama’s reluctance to take the steps he announced Wednesday reflected a president who prioritized his legacy as a commander in chief who ended wars over warnings about the threat that was building in the Middle East. As a result, critics say he contributed to creating the conditions that allowed the Islamic State militants to thrive and move freely across the border between Iraq and Syria.

“The president is a rather reluctant commander in chief,” Senate Minority Leader Mitch McConnell, R-Ky., said Wednesday, echoing the comments of other Republicans responding to a vexing foreign policy crisis that is reaching fever pitch just weeks before November’s midterm elections.

While Obama’s advisers dispute the assertions of their critics, there is little doubt that the president now feels a need to reverse course after resisting the tug of the Middle East.