What do Quakers believe about war and serving in the military? It seems that I once heard that Quakers are pacifists, but I'm not sure if that's just a rumor or not.

What is the Quaker doctrine regarding war and killing people as a soldier in the military?  If they think it is wrong, what is the biblical basis for this belief?

Also, do their beliefs extend to being a soldier in the military under any capacity (such as a chaplain, for example)?
Quakers are considered one of the "peace churches" who oppose war of any kind, and typically refuse to participate in it. However they are also not a denomination which expects everyone in it to do exactly what the church says. See this article for more information. The biblical basis for pacifism is discussed here.

I believe that some Quakers have indeed served in the military as medics. I've never heard of one serving as a chaplain.