In doing so, Sanders drew a sharp contrast with Hillary Clinton, who earlier Wednesday made her own case for supporting the deal.
Sanders did not mention his vote against the war in 2002, but he has frequently cited his early opposition to the bill that authorized military force, and, like other Democratic candidates, has used the issue to highlight differences between him and Clinton.
In the speech, Sanders characterized the GOP as war-mongering.
"I fear that many of my Republican colleagues don't fully understand that war must be a last resort not the first resort. It is easy to go to war. It is not so easy to fully comprehend the unintended consequences of that war," Sanders said. "Yes, the military option should always be on the table, but it should be the last option."
Sanders' remarks came hours after Clinton delivered a forceful yet cautious backing of the Iran nuclear deal in a speech at the Brookings Institution in Washington, saying it helps protect Israel and pledging that, if elected, she would strengthen the security bond between the Jewish state and the U.S.
The Senate is currently debating a resolution to disapprove of the administration's agreement. Forty-two Democratic members of the Senate have come out in favor of the deal, giving President Barack Obama enough votes to prevent a likely GOP-led resolution disapproving of the deal.
"It is my firm belief that the test of a great nation with the most powerful military on earth is not how many wars it can engage in, but how it can use our strength and our capabilities to resolve international conflicts in a peaceful way," Sanders said. "I believe it is incumbent upon us to give the negotiated agreement the chance to succeed."
Clinton's vote to authorize the war in Iraq has nagged her political career for years, most notably during her 2008 bid for president, when then-Sen. Barack Obama, who publicly opposed the war, cited Clinton's support to claim he was better suited to lead on foreign policy.
Clinton has since said her Iraq War vote was a "mistake."