This article is from the The Religious Society of Friends FAQ, by Marc Mengel firstname.lastname@example.org with numerous contributions by others.
Friends have generally refused to fight in wars, in particular
refused the draft, since the mid to late 1600's. As the
"George Fox Song" says:
"If we give you a rifle
will you fight for the Lord?
But you can't kill the Devil
with a gun or a sword."
Friends groups like the Friends Committee on National
Legislation (FCNL) lobby heavily against military involvement
and military spending along with their other priorities.
Friends are also concerned about finding causes of war in our
daily lives -- do you own something that someone else would
kill to have?
Friends organizations (like the Friends Ambulance Unit in
both World Wars) have attempted to reduce the suffering of
wars, and Quaker House near the United Nations is active in
various diplomatic efforts, allowing "off the record"
discussions between parties who don't officially recognize
one another, etc.