Hmm...to quote Janeway, "There are always alternatives to war."
Tell that to the Dominion.
Given that the Federation attempted diplomacy with the Dominion, I would say that they actually attempted the alternatives before going to war.
Voyager, on the other hand, took one look at 8472 and said, "Hey, I have an idea! Let's align ourselves with our mortal enemies!"
The point is, Voyager didn't attempt diplomacy beyond lip service. To quote another Trek film:
That worked out well...I thinkJames T. Kirk: The enemy of my enemy is my friend.
Spock: An Arabic proverb attributed to a prince who was betrayed and decapitated by his own subjects.
James T. Kirk: Well, still, it's a hell of a quote.
The 8472 didn't give them any opportunities for diplomacy, they forced VOY's hand.