I assume two to three antagonists (on the planet) of approximately equal military potential, located on separate landmasses, in possesion of nuclear weaponry assuring the certain destruction of all in the case of heated escalation.
A. Radio Broadcast Message
The antagonists are jamming all radio-wavelengths and filling the channels of communication with white noise.
Broadcasting on all radio-bands, even to the point of increasing the power of the signal to the level that it would damage the receiving equipment, was tried, but the antagonists on the planet did not answer.
The increase of emission of several large broadcasting stations, whose broadcasting now resembled more chaos or a special code resembling chaos, was observed when the aliens set on to the journey from home planet to the planet of the antagonists.
B. Reason for Journey
The Journey was undertaken as an expedition from the planet of the aliens. THe planet travelled to showed civilizational activity due to its output in the electromagnetic spectrum, position to its Star and so forth. Hence it was determined that in the time needed to reach the other planet, the civilization would still be there to contact.
The actual travel was accomplished due to time dilation utilizing a massive black hole, but that will be another question.
Unfortunately, the resource consumption for the journey was prohibitive, so turning back is considered, but ruled out repeatedly by the crew and finally the captain of the expedition.
C. Reason nut Nuked each other yet
On the planet there are several underground rocket silos, highly camouflaged and hidden in mountain valleys and underwater ridges.
The communication between the headquarters and the weapons over close ranges is possible by cable, however atmospheric transmission of signals is highly disturbed by the antagonists broadcasting stations.
While the antagonists could launch (maybe not all but at least some) of their rockets, they instead only keep jamming the antagonists broadcastings from ground stations and satellites.
Assume that our (the aliens) sending of probes, the signal emission—the delivery of the mail has successfully occured. The antagonists have received the codes transmitted and have mastered them.
Now assume we can send both sides the identical request for contact, either with a guarantee of neutrality which is genuine, or with a guarantee of exclusive support which is false.
That means we tell each side that we are addressing the other at the same time, or else assure it that we are approaching only it for contact.
We are not allowed to unnecessarily risk our own lives by falling into a trap, nor are we to provoke the two blocks into an all-out war that would wipe them out.
Which type of message send to both antagonists would have a a higher probability of (successful) contact?