Sen. Lindsey O. Graham (R-S.C.) said President Trump’s Afghanistan announcement was “paving the way toward a second 9/11.” (Jahi Chikwendiu/The Washington Post)

President Trump’s decisions this week to withdraw troops from Syria and Afghanistan, and Defense Secretary Jim Mattis’s resulting resignation, have all but shattered what waning Republican support remained for the president’s global priorities and vision.

One of Trump’s closest confidantes in the Senate, Sen. Lindsey O. Graham (R-S.C.) charged that the president’s Afghanistan announcement was “paving the way toward a second 9/11.” Senate Majority Leader Mitch McConnell (R-Ky.), Trump’s frequent policy partner, said that for Mattis to depart over “sharp differences with the president” was “regrettable.” And one of Trump’s most diligent backers, Senate Armed Services Committee Chairman James M. Inhofe (R-Okla.), said it was “inappropriate” for Trump to have ignored his national security advisers.

Several lawmakers even suggested Trump lacked the credibility to make such decisions, having never set foot in a conflict zone. “It would be good if he visited the area,” Sen. Roger Wicker (R-Miss.) told The Washington Post, noting he was “gravely concerned” about the president’s announced intention to withdraw troops from Syria and Afghanistan, and was “urging the president to revisit the decision.”

But as political allies turn into critics and boosters express their disappointment with him, the GOP is at a loss for exactly how to challenge the president, knowing they have few tools in their legislative arsenal to steer the commander in chief off a course he appears determined to pursue.

“I’m not sure what options we have,” Inhofe said.

“What would you suggest?” said Sen. Deb Fischer (R-Neb.), a member of the Armed Services Committee. “The president is the commander in chief and he will ultimately make those decisions; it’s my job to either support those decisions or to raise concerns.”

Fischer said she was “disappointed” with Trump’s military moves and the resulting fallout with Mattis.

Trump’s new policies are consistent with the pro-disengagement views he espoused on the campaign trail. Yet they are the latest in a long line of foreign policy decisions most congressional Republicans have found shocking.

Trump has inspired protests from within his own party over his seeming embrace of Russia — in the face of its election interference — and Chinese companies seen as national security threats. He has also been challenged from various corners of the Republican establishment over his aggressive approach to allies, whether by making demands of NATO partners or upending trade and tariff policies.

Most recently, Trump’s party has revolted over his defense of Saudi Arabia’s crown prince, Mohammed bin Salman, despite evidence he ordered the murder of journalist Jamal Khashoggi. Earlier this month, Senate Republicans rallied in opposition, rebuking the president and the Saudis by voting to end the United States’ continued participation in the Saudi-led war in Yemen.

Yet Trump’s Syria and Afghanistan decisions strike at the heart of mainstream Republicans’ long-held belief that a U.S. military presence is necessary to maintain a modicum of stability in areas where terrorist groups have taken hold. Republicans warned the Obama administration not to withdraw troops from Iraq; now, they say Trump is repeating his predecessor’s mistakes.

“That would be worse than anything Obama did, and that is saying a lot,” Graham said of Trump’s desire to reduce the U.S. military footprint in Afghanistan by as much as half.

Early next year, senators will have a chance to vet Trump’s eventual nominee to replace Mattis as defense secretary — an opportunity, should Republicans leverage it, to insist that whoever takes over at the Pentagon will check Trump’s global strategy.

Graham is one of the few GOP senators who have suggested proactive steps in the meantime to challenge Trump’s new disengagement strategy. He stood shoulder to shoulder with Democrats this week to stump for a sense of the Senate resolution calling on Trump to consult with his national security team before he makes any future decisions regarding troop deployments. On Friday, he called for hearings in the Armed Services Committee “like right now, about Syria” and said he wants “to hear from Mattis” about everything that has transpired.

Others point out that such measures wouldn’t be binding on the president, and questioned what Mattis would even tell senators as he heads for the exit.

“Secretary Mattis laid it out well in his letter,” Fischer said. “To do a post-mortem after you lose your job, I don’t think that’s the way to go.”

Inhofe, the Armed Services Committee chairman, said he thought it would be more appropriate to examine the Syria and Afghanistan strategy in the Senate Foreign Relations Committee.

The foreign relations panel traditionally has purview over authorizations for use of military force. But the current debate is less about limiting Trump’s use of troops than encouraging him to keep them in the field — and no authorization can force the president to do that.

“You’ve got some people who want to wade into [such] discussions. . . . I don’t think there’s any consistent strategy here, though,” said Sen. Thom Tillis (R-N.C.), a member of the Armed Services panel. “I don’t see any legislative action.”