I believe that war can be justified, but only in extreme circumstances e.g. In World War II. If Hitler had not been stopped then the effect on the world may have been even more horrific. The loss of human life would have been far greater if Hitler had progressed to take over the rest of the world and spread the ideas behind his regime to everyone else. Even though this war cost at least 50 million lives it is almost always considered just and fair.
War is almost always a means of forcing a political policy. If no other option exists then it is justified to go to war. However, I believe that the relative morality of war is usually defined years afterwards.
The justification of war should be considered in terms of the ability to conduct the war with the minimum number of casualties and in a short time.
Of course it hardly ever is that simple. There have been many wars that have been entirely just but have had a worse effect than the thing they have been trying to combat.
The difficulty of deciding if war is justified falls on our representatives, however, the final decision of whether we should go to war or not, I feel, should lie with the people - for it is the people who will be fighting the war and the people who will lose friends and family through it. It should be up to the people to decide if the risk is justifiable- not the politicians who sit in offices, with bodyguards, away from any real danger.
I believe that we should not go to war unless it is shown that not doing so would cause more suffering and more death than doing so. But sometimes, this doesn't work either.