Quote Originally Posted by abou
In my opinion, Antiochos was probably tired. He was quite old by now and had been campaigning constantly since he ascended to the Seleukid throne. Only two people come even close to this: Alexander, who didn't live very long, and Caesar who took many months on a vacation sailing on the Nile with Kleopatra probably due to exhaustion. Antiochos was also aware of the what damage the four years of fighting with Achaios had caused to the region. In the end, he went with the Treaty of Apameia, which had its pros and cons for both sides and was a far more peaceful solution.
I think that Antiochos must have had serious financial problems. War wasn't cheap even for Seleucids. AFAIK when two ancient empires fought each other the first major battle was decisive because army upkeep was so high. One had to win and plunder enemy camp to pay the army. Looser was broke and had to sue for peace.

The Romans are another matter. They didn't pay for solders equipment (until Marian reforms of course) paid much less their soldiers so money wasn't that big problem for them as for other countries.

What is more Magnesia wasn't first defeat of Antiochos. He lost one or two battles trying to stop Romans in main Greece. So after Magnesia his army had very low morale, he had no money, as Abou says was tired and probably
he was afraid that after defeat at Magnesia some more energetic governors could think it is a good time to become independent. Unfortunatelly Seleucids Empire wasn't very stable.