In the trade, this type of treatment is known as "being hung out to dry." Your associates felt that it would be politically incorrect to agree with you, so they jumped ship. You have every right to feel betrayed. After all, your fellow employees breached an agreement and acted in a self-serving, unfair, and unkind manner.
Although it is tempting to tell them that you are disgusted and disappointed with their behavior, all that will do is generate hostility and defensiveness. Instead of telling them anything, you should ask them what happened. Be prepared for a combination of side-stepping, rationalizing, and double talk. In due time, there will probably be apologies and fence-mending.
Regardless, you should meet with your manager to elaborate on what happened in the meeting. Let him know that the entire group was prepared to talk about dealings with the other supervisor, but there was a last-minute change of heart on their part. Rather than complaining about the supervisor, you should now describe the operational problems and then suggest some specific ways for the departments to work more productively together. Although there may well be a problem with the supervisor, one has to wonder whether how much of the problem is your co-workers.
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