1. Quakers are a religious society with a stance against participating in war
Quakers are a faith community. They have a collective position that participating in warfare is wrong and against the teachings of Christ Jesus. This is a clear and longstanding corporate witness growing out of Quakers' understanding of Christianity.
2. Individuals must still make their own choices
Even when the teachings are clear, each individual has to decide for himself or herself what is right action in a particular context.
Even though Quakers opposed all war, many individual men signed up for the US Civil War. Many of these volunteers were read out of meeting; and a goodly portion of them were accepted back into fellowship afterwards, especially if they expressed contrition. Even if the official witness of Friends was against the war, Friends supported an end to enslavement, and as a result many could understand why an individual might choose to sign up, even if it was officially outside the Quaker testimony.
Later, many young Quaker men even signed up for World War I, to the consternation of many elders, including Rufus Jones. The American Friends Service Committee was founded in 1917 in part to provide young Quaker men with meaningful service opportunities outside fighting in the military. One option was the Friends Ambulance Corps.
3. An individual may find a different answer than society at large, and need not have all the answers for everyone
Today, most people think of Quakers as not just refraining from serving in the military themselves, but also as people who act to stop war before it begins. This is a relatively modern approach.
Many Quakers involved in peace work have analyzed past wars, diplomacy, and popular movements to understand strategies and tactics to prevent war in the future. There are too many examples to name here, so I'll list just one information resource: Quaker George Lakey led the creation of Swarthmore College's Global Nonviolent Action Database, where you can learn more about popular nonviolent movements. (Side note: And there can be practical benefits to a peaceful approach: According to Waging Nonviolence, in researchers Erica Chenoweth and Maria Stephan found that nonviolent struggles were successful significantly more often (56%) than violent ones (26%). See their book Why Civil Resistance Works.)
In some cases, Quakers have identified the failure by "good actors" to act in a timely way to prevent demagogues and dictators from rising to power. Once those moments are past and it's "too late," war may seem inevitable in hindsight. For example, theologian Walter Wink faulted the churches in Germany for not speaking out against Hitler and the Nazi putschists in the early 1930s. For another, peace activist David Hartsough and five colleagues were arrested in Kosovo in 1998 while there to support the mass student nonviolent movement against the Serbian dictatorship. Despite the pleas of the students, Hartsough, and others, the nonviolent resistance was not supported by other Western nations and so was crushed, leading to genocide, internecine warfare, and US and NATO bombardments when it was "too late" to support the movement.
So one may intellectually concede that wars of international aggression appear to have no solution besides war -- when they reach that point. They are not inevitable, however, if people of good will and conscience resist before that point.
Finally, even when one or more nations begin warfare, people of good will and conscience have a choice. They can refuse to participate, no matter how just the cause may appear, or seem to appear, in order not to damage their own spiritual well-being. Such a person can know in her or his heart that this is the right action for him or her. Even when the larger societal answer seems to point to war as the answer, the still small voice can still tell that individual, "War is not the answer." And if everyone lived that way and followed that voice, the world would be a different and better place.