(covers information from several alternate timelines)
Pacifism was the ethical position that violence and war were immoral. Strict pacifists refrained from all forms of violence, while restricted pacifists made some exceptions, for example, in cases of self-defense.
One who attempted to maintain peace was known as a peacekeeper. In 2258 of the alternate reality, Admiral Alexander Marcus questioned whether Khan Noonien Singh had claimed to be a peacekeeper, saying he and his cronies were in fact war criminals. (Star Trek Into Darkness)
In the late 1930s of an alternate timeline created by Leonard McCoy's escape through the Guardian of Forever, Edith Keeler lead a pacifist movement that delayed the United States' entry into World War II, thereby giving Germany the time they needed to develop the atomic bomb first and win the war. (TOS: "The City on the Edge of Forever")
- Vulcans (insofar as they followed the teachings of Surak)