By Kate Sullivan, CNN

(CNN) -- Republican Sen. Lindsey Graham called President Donald Trump's decision to immediately withdraw US troops from Syria a "disaster" and "a stain on the honor of the United States."

In a speech on the Senate floor Wednesday night, Graham, one of Trump's closest allies in Congress, called the move "disastrous to our own national security" and a decision that was made "against sound military advice." Graham said this decision would make Americans around the world and here at home "more at risk, not less."

The South Carolina congressman said he was shocked by the decision and characterized the move as a betrayal to our allies in the region.

"We have been dishonorable. This is a stain on the honor of the United States," Graham said.

Graham, who said he just got back from a trip to Afghanistan, said: "Having been in the military for quite a while, I'm very aware of the sacrifices required to go overseas and serve in Iraq, Syria and Afghanistan."

The senator said US deployed forces are necessary to keep Americans safe and act as a "virtual wall." He said they are the "best hope we have of stopping another 9/11, protecting ourselves and our allies."

"I'm not saying we need to be in Syria forever. I'm saying now's not time to leave," Graham said.

Graham was one of a group of senators who wrote a letter to Trump on Wednesday night asking him to change course.

Republican Sens. Joni Ernst, Marco Rubio and Tom Cotton; Democratic Sen. Jeanne Shaheen; and independent Sen. Angus King sent the letter.

In it, they wrote, "If you decide to follow through with your decision to pull our troops out of Syria, any remnants of ISIS in Syria will surely renew and embolden their efforts in the region. However, ISIS is not the only threat. The brutal dictatorship of Bashar al Assad continues to weigh heavily upon the Syrian people, and we fear that a withdrawal of our troops may embolden Bashar al Assad to take further actions to solidify his power."