The large crowd stood in the freezing cold to hear speakers like Juan Cole, a University of Michigan professor specializing in Middle East history.
He called targeting Iranian Major General Qassem Soleimani an unnecessary act of war.
"To be against war with Iran does not mean being pro-regime," Cole said. "It doesn't mean we approve of the government of Iran."
Some students said they are not politically active, but the threat of a war with Iran brought them out to the rally.
U of M Sociology student Shelby Christensen says she came to learn more about the history of U.S. intervention in Iran, Iraq, and the Middle East.
"This is pretty awful what we're doing," she said. "It's not what I like to see, and it's not what anybody here likes to see."
Earlier in the day, Trump said Iran appeared to be "standing down," after it launched missiles toward bases in Iraq housing American troops.
The missiles were in retaliation for Soleimani's death.