An act of war is whatever the participants define as an act of war. The US might have considered it an act of war by the USSR when a simple barbed wire fence was erected in turf that belonged neither to the US nor to the USSR... Political convenience dictated otherwise.
The Dominion defined the mining of the wormhole mouth as an act of war. It was, then.
However, the Federation never defined the blowing up of the Odyssey
as an act of war. They were quite willing to tolerate it, and never went to war about it. Nor did they consider the attempted destruction of Bajor's star an act of war. Admittedly, in the first case it was initially difficult to contact the culprits, and in the second it was difficult to demonstrate their guilt. Nevertheless, the war did not start over either of these acts.