The chairman of the Senate Foreign Relations Committee revealed on Friday that the U.S. military was planning a “major clearing operation” targeting ISIS before President Donald Trump decided abruptly this week to withdraw U.S. forces from Syria.

“One thing that hasn’t been reported is, we were six weeks away from a major clearing operation that has been planned for a long time. I got briefed on this a year ago—with ISIS in the Euphrates River Valley,” Sen. Bob Corker (R-TN) said Friday on Capitol Hill, referring to the area where ISIS leader Abu Bakr al-Baghdadi is believed to be hiding.

Trump’s decision, which at least partly led to the resignation of Defense Secretary Jim Mattis, has rattled congressional Republicans, who have questioned the wisdom of withdrawing from Syria before ISIS is fully eradicated. In defending his decision, Trump claimed that the extremist caliphate has been defeated, but Sen. Lindsey Graham (R-SC), a top Trump ally, called that claim “fake news,” and said America’s adversaries will benefit from Trump’s order.

Aaron Stein, director of the Middle East program at the Foreign Policy Research Institute, said he was unfamiliar with any specific U.S. operation planned in the area but that further cleanup efforts against the extremist group seemed logical.

“I’d assume this was a final push in the Middle Euphrates River Valley to clear the small ISIS-held villages out near the Iraqi border,” Stein told The Daily Beast.

Corker said “at least 20,000” ISIS fighters are still in the region. The “major clearing operation” Corker referenced “would have changed the dynamic tremendously” in Iraq and Syria, where ISIS has lost significant territory in recent years. He later clarified that the operation was intended to take place in “six to eight” weeks.

“So to pull the plug—it’s just hard to understand,” Corker said of Trump’s decision to pull U.S. forces from Syria. “If you want to move away from Syria, why wouldn’t you do it after you’ve done the work that you’ve been planning for so long? Why would you pull the plug in advance of that? That’s the part that I think is heartbreaking to those in the military that are aware of what we were getting ready to do, to those of us who supported the clearing of ISIS and the things that have happened on the ground in Syria.”

From Trump critics like Corker to close allies like Graham, Republicans have spoken out uniformly against Trump’s decision, which lawmakers have said was reached without consulting with congressional leaders and against the advice of the president’s top national-security aides. In particular, Republicans compared Trump’s directive to former President Barack Obama’s withdrawal of U.S. troops from Iraq in 2012, which they say led to the rise of ISIS.

When asked about his colleagues’ willingness to criticize the president when they have been unwilling to do so on other issues, Corker said: “It’s one thing to have issues on the domestic side. When you start willy-nilly foreign policy moves that are against U.S. interests, that is a wake-up call for people in the Senate.”

Corker intended to raise the issue with Trump earlier this week, but the president apparently canceled a pre-planned meeting with Corker while the senator was already waiting at the White House. U.S. troops have already begun leaving Syria, Corker said.

“It’s a place of not of anger—it’s of sadness for our country. And then to have such disrespect for our coalition partners where we don’t even talk with them in advance, and simultaneously we’ve got tariffs on them,” Corker added. “So we’re in a really bad place right now as it relates to foreign policy.”