Good point Cicci. I have always felt the same thing. My take on it was Tony really had no choice. Philly was looking for a reason to go after the family; it just took him a long time to lead up to it. After he officially got the big seat, as is the case most times, the power went to his head. Im sure it was something that was always in the back of his head, but it was inevitable.
Tonys cousin wacked Phils kid brother, and he didn't even let him get reprisal. He beats the shit out of phil when phil isnt forthcoming with the money he owes Tony. Those two things alone shoulda gotten Tony wacked.
Tony reacted because he had it on good info they were coming after him, so he did what he had to do. Did he actually think he was going to win? Probably not, but by then it was too late.
Tony had no choice, it was either kill or be killed. Eitehr let new york trample the family and take over, or go down swinging.....