What important role did Joshua Chamberlain play?

He was a professor at Bowdoin College in Maine, and joined the Union Army as an officer. But what most people know him for is commanding the 20th Maine Regiment at the Battle of Gettysburg. He earned the Medal of Honor there- his regiment was the edge of the Union army, and had to defeat several Confederate assaults. At one point, his troops ran out of ammunition and he had to call for a bayonet charge, which successfully drove back the Confederate troops.

If his regiment hadn't held the line there, the Confederates could have broken the Union flank, and if that had happened, the Battle of Gettysburg- and even the whole Civil War- would have ended very differently.