I had a similar situation in the Napoleonic Mod. I was England and had become involved in a very long campaign against the French after they had destroyed my Dutch Allies and had taken Friesland from me. I counterattacked into Friesland and was joined by a fairly large Danish force that said that they were my allies. I had been allies with them almost since the beginning of the game and had helped them lift sieges in the past so I thought everything was good between us. I began to form my lines and began a arty barrage on the French army that was on top of a small hill. I wasn't really paying much attention to the Danes as they began to march forward towards the French lines. I thought this was great, I could sit back and shell the French, the Danes would march in and cause even more death and destruction and then I could send in my cav to sweep the French off of the field. The next thing that I know, the Danes wheel around and form right in front of my lines and open fire on me. Most of my arty was destroyed since it was in front of my lines while I tried to get formed. They moved forward and engaged my front lines and began to actually push me back a little at first but I began to gain the upper hand thanks to my KGL. Then I notice that the French have left thier hilltop position and have begun marching down the hill towards me. They hit my right flank with everything that they had cav wise and my line crumbled. I decided to just give it up and retreat so I selected all but two KGL units and withdrew them from battle while my last two units fought a rearguard action. The Danish and French Cav charged my retreating troops and swept them from the field. I entered the battle with 980 troops under the command of a 5* general and managed to get my general and two half units of light cav off of the field. I ended up losing about 920 men from that army and the French kept the province. The next turn, I moved every availible army stack that I had into the Danish territory and crushed them in about three turns of fighting and sieges.

I had never seen this much of a backstab before this