Diplomacy had failed, no compromise could be reached. As a last ditch effort to salvage the de-escalation talk and pave the way for peace, a human had been called in. They had all gathered to hear his plan in the large auditorium, a neutral space station for both sides.
The strange human looked tiny standing in the middle. He begun to speak, "It is clear to me that no simple peaceful solution can be found and escalation is currently fast progressing towards war. Therefore I have created some simpler doomsday weapons for both sides. These should be plenty to totally eradicated each other. Creating a balance without any additional forces required. Any questions?" Dead silence, most weren't sure what they had heard. Some were and their play dead instincts had kicked in, littering the floor.
One brave voice decided to speak up, "Ehm, yes. What precisely do you mean by doomsday weapons?"
"Glad you asked! The forefather for these were developed when we were still a one planet civilization, in a world war on my planet and were used-"
A voice interrupted his speech, "Wait, world war? And did you just say you used doomsday weapons on yourself?"
"Yes, the earliest iterations of theses weapons on a tiny scale. Proper ones capable of a more complete doomsday scenario where developed some years later."
Another brave voice sounded from the audience, "You keep saying doomsday, what do you mean?"
"The complete destruction of all larger life on a planet. Since you both are single planet species that should be plenty."
Most had difficulty believing what he was saying. Others had regained control of their bodies again, only to hear the continuation of the conversation and promptly collapsed. "So your plan is to give both sides the ability to completely eradicated the other... are you mad?"
"Yes, precisely! MAD, Mutually Assured Destruction. Firing of these weapons from one side will leave plenty of time for the other to retaliate. Leaving both sides worse of."
"Dead."
"Yes."
One of the highest ranked commanders in the room stood up, "All in favor of baring entry to all humans from both our systems?"
"Ai," came the unanimous reply from the hall.
"Wait, it will work! We have done the same thing. Its not perfectly safe but-" He began to say many other things, but the guards had gotten to the podium and started to drag him towards the exit.
Later a compromise of a defensive alliance to counter the human threat was created. Baring entry from any and all humans. With the threat of their total destruction should they not comply.