"Milosevic and the world now knows that we will not let racial genocide go on without challenge," he said. "We will not see the values of civilisation sacrificed without raising the hand of justice in their defence."
His remarks will be seen as a clear sign that the Kosovo conflict marks a decisive shift in the role of Nato from a defensive alliance to a policing role.
Mr Blair said the international community had faced a "moral choice" - whether to let the "barbarism" happen or to stop it. In doing so, the world could not prevent "death and destruction" to many people.
"War can be necessary sometimes to uphold civilisation, and this one was," he said. "Nothing we do or say can compensate for the loss of loved ones who died in the conflict, but we can say they did not die in vain."
Mr Blair said that with the backing of the United Nations, the entry of military forces into the beleaguered province would begin "shortly". He added that the priority was to see refugees returning to their homes.