RMB click-the-stack and drag will show their _projected_ path. Cancel the line with LMB. Be forewarned, I have done this and seen my army fly off into the wilderness. I've only seen it happen a few times, but it seems if too many stacks (not necessarily full ones) get bunched up in an area, the pathing will show line-X, but when it comes to moving, it somehow decides it's too crowded and chooses line-Z, which is usually a roundabout way. If things look dicey, i hit spacebar (for slow animated moves) and then try the movement, ready to hit backspace to stop them if they suddenly take their "shortcut".
Bookmarks