The terrorist attacks in Brussels is putting President Obama's controversial approach to combating the Islamic State back under the microscope, opening him up to renewed criticism just as he's trying to drive his foreign policy agenda and his poll numbers reach a three-year high.

Republicans roundly criticized Obama for continuing his historic trip to Cuba and Argentina after the attack, but even before Tuesday's tragedy, experts were handing out harsh grades for Obama's foreign policy in the wake of a lengthy, self-assessment published in The Atlantic earlier this month.

Congressional Republicans and many experts have accused Obama of taking only half-measures against the Islamic State and Syrian President Bashar Assad.

"We've got to give our pilots and our commanders already on the ground the flexibility they need," House Foreign Affairs Chairman Ed Royce, R-Calif., said in response to the Brussels brutality, for which the Islamic State has claimed credit. "We've got 180,000 Kurdish forces that are on a 650-mile front with ISIS and they're fighting with 40-year-old weapons. We've got to get them the mortars, the artillery, the anti-tank weapons to win that fight."

"Unfortunately, the White House still doesn't have a plan to get the job done," Royce said, echoing a complaint of Senate Armed Services Chairman John McCain, R-Ariz., Senate Homeland Security Chairman Ron Johnson, R-Wis., and many others.

Obama made clear on Wednesday that he's not interested in how the GOP is grading him, and has no plans to change course.

Speaking in Buenos Aires on the second-leg of his Latin American trip, Obama said ISIS has "hunkered down" because of the U.S.-led bombing campaign and strategy.

"I've got a lot of things on my plate, but my top priority is to defeat ISIL and eliminate the scourge of this barbaric terrorism that's been taking place all around the world," he added.

Obama boasted in the Atlantic interview that when it comes to the Syrian civil war, he broke with the "Washington playbook" by not attacking Assad after he used chemical weapons against his citizenry.

But some of the criticism is coming from Obama's own corner. Obama's former Defense Secretary Robert Gates has said Obama and his advisers are poor strategists.

"You know, the president is quoted as having said at one point to his staff, 'I can do every one of your jobs better than you can,'" Gates said on MSNBC back in January. "One of the greatest weaknesses of the White House is implementation of strategy — is difficulty in developing strategy and then implementing that strategy."

More broadly, experts, Republicans and some Democrats pan Obama's entire view of the Middle East. The Center for Strategic and International Studies' Anthony Cordesman took Obama to task last week for abandoning Afghanistan.

"The Obama administration's lack of focus on the Afghan War is symbolized by the fact that it is no longer even listed as one of the "Top Issues" on the Department of Defense's website," he wrote March 16. By starving that effort of necessary resources, Cordesman argues that Obama will leave his successor a no-win situation there.

"Cutting Afghan forces to critically low levels before [Obama's successor] takes office will hand the next president the worst possible beginning: A defeat that the new leader has no opportunity to avoid, created by a situation there was no opportunity to correct."

To be fair, not everyone gives the president and his "Obama Doctrine" low marks.

"I give Obama reasonable marks for carefulness and strategic thinking," the Brookings Institution's Michael O'Hanlon recently assessed. "He has been a proficient commander in chief, and it is possible that we will someday badly miss his judiciousness."

But a former State Department aide who focused on Middle East policy said Obama's obsession with avoiding President George W. Bush's mistake of invading Iraq led him to two major errors in judgment that helped give rise to the Islamic State.

"The first mistake was Obama's retreat from Iraq — the withdrawal not just of U.S. forces, but even more so of diplomatic energy and leverage," Tamara Cofman Wittes, now of the Brookings Institution, recently wrote.

"Likewise, Obama's read of the Syrian conflict as holding only narrow implications for American interests was a signal failure to learn the lessons of the 1990s and recognize the risk that Syria's civil war could spill over in ways that directly implicated U.S. interests," she stated.