The Seleucid and Ptolemaic Empires are already at war. However, I'm afraid the balance of power between those two mighty empires may be in danger of collapsing.

Seleucus is at war not only with Ptolemy but with the Parthians and the Greeks at Byzantium. That is three distinct theaters. As you can see in the World Overview History (available at your local library), Seleucus has recently lost ground to both the Greeks and Ptolemy, while he has managed to regain some lands previously lost to the Parthians. What's worse, Ptolemy has conquered large parts of Syria, and is close to Antioch, the capital of the Seleucid Empire. Ptolemy, on the other hand, is only at war with the Seleucid Empire and Macedon, the latter of which is effectively finished.

I believe that the Selecid Empire is in danger of a serious defeat, or even a complete collapse, at the hands of the Ptolemaic Empire. This would seriously upset the balance of power in the east. If the Seleucid Empire collapses, Ptolemy could, in addition to holding Egypt and Thrace, also gain control over large parts of Asia, Syria, and even Persia.

It is for that reason that I believe that we should support the Seleucid Empire in this conflict.

I am not advocating an immediate war with Ptolemy. But, we should certainly not aid him further. If anything, we should aid Seleucus however we can.

We should hold on to Byzantium as it is an important strategic point, and would prevent Ptolemy from gaining another access route to Asia. I doubt that Seleucus would be able to defend, he's even losing ground to the Greeks. Nicomedia we should give back to Seleucus once it is captured from the Greeks, perhaps in exchange for Alliance.

Debeltos is not too important, and if Seleucus does not want it, then it could perhaps be sold to Ptolemy. We could also keep it, without any special defenses, other than a normal city garrison. Ptolemies of Thrace have not attacked it even though it's Macedonian; I don't think they're interested in it. It would be more useful for us, as it can trade with 4 or more other cities on the Black Sea.

In any case, whatever we do, we must not give further aid for Ptolemy, or seek an alliance with him. Instead, we should support his rival, for the reasons outlined above.