Quakers have historically held to the peace testimony. The implication of holding the belief that “There is that of God in everyone” is that most Quakers feel that war is absolutely wrong and that individuals should not support it, under any circumstances.
The peace testimony is manifested in a position called Conscientious Objection to military service. If all people are the beloved children of God, and if there is that of God in everyone, we cannot accept war. That is, a nation cannot impose its will on other nations by force and violence, by killing the military force of the enemy and destroying their property.
The Quakers have mostly held to this peace testimony over the centuries, and (along with the Mennonites and the Church of the Brethren) are considered the Historic Peace Churches. This is Quaker history, although in each war as it comes along, individual Quakers have to consult the Inner Light and make a conscious decision about whether they will serve.