It is also rather likely that Starfleet simply wasn't ready for war yet. We heard later on that Starfleet had thousands of ships - but several episodes had established that it was impossible to bring such numbers to the vicinity of DS9 in any appreciable time. Indeed, it was only ever possible to summon less than a dozen ships in peacetime, even when the reason for summoning was that a foreign fleet numbering hundreds of ships was going to war! ("Way of the Warrior", "The Die is Cast") We also failed to witness any appreciable shipbuilding during the war: all ships witnessed had registries lower than those of ships seen before the war, and the one documented newbuild, USS Sao Paulo
, was a rather small unit.
It thus would be easy to speculate that Starfleet can go to war only by waiting for two years. Right after the events of "Jem'Hadar", the call to war would have been covertly sounded, and thousands of ships would have begun to gather. But they would be gathering from that uniquely UFP thing, deep space exploration errands, and would not reach home until months or years of travel.
Thus, delaying and delaying again would be the only way for the UFP to reach the balance in numbers required for successful waging of war. And that would mean delaying even when the Dominion started pushing forces into Alpha, until the looming numerical defeat tilted the balances and made an attack, however premature, the better chance for survival.