If you think it was an impulsive decision, Use of Time, you haven't paid attention.
When Dukat makes the announcement he actually states that the alliance was made after
he "conducted long, secret negotiations". Some planning definitely went into this. (Having said that, he was by no means beyond impulsive acts on occasion, especially when he was being thwarted, or became infuriated.)
And think of - if it had all gone to plan, he'd have become the national hero. Plus the alliance allowed him to take up position as Cardassia's leader
. How could a man as power-hungry as that resist? It's the ultimate prize!
DISAGREE ENTIRELY with above remark re.: Damar and Ziyal.
From where Damar was standing, he didn't murder some innocent girl, he executed a traitor. She collaborated with the enemy, she shared responsibility for the weapons system being down when it was needed in defence of the station. She betrayed her father and so cost him, and by that token Cardassia, an historic victory. Dukat should have recognized who was really loyal to him at that point, and come back to his senses. He should have thanked Damar if anything.