A person who does not believe in war for any reason and believes in peaceful solutions only is known as a pacifist.
Explanation:
A pacifist is one who tries to avoid war and rather solve the problems by introducing possible alternative peaceful solutions to. There are two broad types of pacifists. One is of absolute pacifists and the other is of conditional pacifists.
An absolute pacifist believes no matter what the situation is, there should be no war. A conditional pacifist accepts that there can be instances in which war is the only way to solve the problem.