I found this organization Soldiers Angels maybe there are others. Outreach to soldiers is the best way to persuade them to stop killing civilians. Maybe they will even begin fragging (killing their officers) like in Vietnam. It's worth a try. At least send your newly adopted soldier some real news instead of the Stars and Stripes propaganda they hear 24/7.
Here's a suggestion: Make nice the first time you contact your soldier. Then send anti-war information or whatever. Once you have the address/name and build a relationship your mail will get through. The first time it may not if it's clearly anti-war that's why it might work best to make nice at first.