I was recently utterly shocked by the AI too. I was playing a migration campaign as Pergamon with Epeiros, and I had already conquered Mytilene and Nikaia. Soon the Seleukids declared war on me by besieging Pergamon. I took my main army (in Nikaia at the time) and lifted the seige, and then went south and besieged the tiny garisson at Sardis. Though I was almost positive they would decline, I offered the Seleukids a ceasefire in exchange for Sardis, and they accepted!