Quakers believe that war and conflict are against God's wishes and so they are dedicated to pacifism and non-violence. And from a practical point of view they think that force nearly always creates more problems than it solves.
We utterly deny all outward wars and strife and fightings with outward weapons, for any end or under any pretence whatsoever, and this is our testimony to the whole world. Quaker statement to King Charles II, 1660
A good end cannot sanctify evil means; nor must we ever do evil, that good may come of it. William Penn, 1693
War, in our view, involves the surrender of the Christian ideal and the denial of human brotherhood. London Yearly Meeting, 1916
Christ demands of us that we adhere, without swerving, to the methods of love, and therefore, if a seeming conflict should arise between the claims of His service and those of the State, it is to Christ that our supreme loyalty must be given, whatever the consequences. London Yearly Meeting, 1915
Many conscientious objectors (those who refuse to join the armed forces) are Quakers, but Quaker pacifism is not simply the refusal to fight, it includes working actively to bring about or preserve peace, by removing the causes of conflict.
Quakers, like other pacifists, are sometimes accused of being willing to give in to evil regimes rather than fight against them. They disagree, and say that they fight by non-violent means.
All forms of non-violent resistance are certainly much better than appeasement, which has come to mean the avoidance of violence by a surrender to injustice at the expense of the sufferings of others and not of one's self, by the giving away of something that is not ours to give. Kathleen Lonsdale, 1953
Quakers are not just opposed to war, but to all forms of violence. George Fox was personally opposed to the use of violence. He refused to defend himself when he was attacked and often, when the violence was over, had kind words or actions for his attackers.