Sen. John McCain (R-Ariz.), as the new chairman of the Senate Armed Services Committee, appears prepared to push for a more aggressive policy. (Gary Cameron/Reuters)

In May 2013, Sen. John McCain caused a stir when he took the risky step of venturing briefly into war-torn Syria to meet with opposition leaders whom he and many other Western backers considered the best hope for toppling Syrian President Bashar al-Assad.

Nearly two years later, 12 of the 15 Syrian commanders McCain met on the trip are dead, proof in the senator’s eyes of President Obama’s failed approach to the conflict spreading across the Middle East.

Now, as chairman of the Senate Armed Services Committee, McCain has the chance to amplify his critique of Obama’s handling of Iraq and Syria and, by doing so, test the panel’s influence over military policy and operations.

“We are probably in the most serious period of turmoil in our lifetime,” said the 78-year-old Republican from Arizona, whose control of the committee is the culmination of decades of tenacious advocacy for a muscular foreign policy. “Everything I’ve predicted, unfortunately, has come true, whether it be in Iraq or whether it be Syria.”

McCain, speaking in a recent interview, sees no shortage of defects in the foreign policy record of the man who edged him out in 2008 to become commander in chief. Beyond the Middle East, McCain has characterized Obama’s response to the conflicts in Ukraine and Afghanistan as weak and inadequate.

Iraq is a particularly compelling cause for McCain, a proponent of President George W. Bush’s troop surge during the last war there. Today, McCain argues that Obama’s withdrawal of U.S. troops from Iraq in 2011 set the stage for Islamic State militants to take over much of the country in 2014.

“We’ve got thousands of foreigners over there in the largest caliphate in history,” he said. Despite months of airstrikes by the United States and its allies against the group in Iraq and Syria, McCain said, “they’re not losing.”

In recent months, the senator has advocated expanding the U.S. force in Iraq, from about 2,300 now to 10,000 to better help Iraqis combat the Islamic State. He and Sen. Lindsey O. Graham (R-S.C.), whom McCain is urging to run for president in 2016, also are calling for U.S. service members, mostly confined to bases and headquarters, to be sent closer to the front lines to direct airstrikes or take other steps to aid local troops struggling to expel the well-armed militant group.

The lawmakers want to establish safe zones or no-fly zones in neighboring Syria and expand aid for moderate Syrian rebels to help them fight back against Assad, who appears buoyed by the months of U.S. and allied strikes against the Islamic State in Syria and Iraq.

Obama has ruled out sending troops back into combat, but U.S. service members are inching closer to the fight as they begin their renewed mission in Iraq. Military leaders such as Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, have suggested they might recommend that U.S. troops take on expanded activities as the battle for Iraq unfolds.

From his new pulpit, McCain probably will attempt to tease out hints of such recommendations from uniformed officials who appear before the committee in coming months. As Armed Services chairman, he controls the pace and topics of oversight hearings and will determine which witnesses provide their views.

“That’s a huge arrow in his quiver he didn’t have before,” said Shawn Brimley, a former Pentagon and White House official who is at the Center for a New American Security. “There will be a heck of a lot attention on current dynamics in the Middle East.”

Perhaps even more significant, McCain will be able to shape the annual defense authorization bill. The measure — which can run hundreds of pages and contains provisions on items as varied as overseas operations and the military prison at Guantanamo Bay, Cuba — is one of the few pieces of major legislation that passes reliably each year.

In this year’s bill, for example, lawmakers authorized the administration to start a program to train moderate Syrian rebels to fight the Islamic State.

McCain’s staff will lead the drafting of the Senate bill, effectively setting the parameters for subsequent debate among lawmakers.

“He’s acutely aware of all the advantages that the chairman has, and I expect he will use them to the full,” Andrew Philip Hunter, a former Pentagon official now at the Center for Strategic and International Studies, said of McCain.

McCain thinks that recent events such as the Islamic State’s beheading of Western hostages and the attacks in Paris have increased support for a muscular response to extremism overseas. But support for a major American military return to the Middle East appears limited, even within a Republican Party that is deeply divided on foreign policy issues.

Although Congress controls the Pentagon’s purse strings, that power appears most effective in constraining military action rather than compelling it. And despite forceful advocacy from McCain and others, there appears to be little appetite in the White House to expand the military mission against the Islamic State, let alone pick a fight with the Assad regime.

As McCain noted wryly, even a powerful Armed Services chairman can exercise only limited influence over military operations that are given ultimate approval by the president.

“By having this position, I am able then to, I think, contribute to our nation’s defense in a way which otherwise would not be possible — particularly since I am obviously not the choice of the American people to be their commander in chief,” he said.