Not all wars can be justified, but some definitely can. WWII I think was probably one of the more justifiable wars in human history. Hitler was a mad man, and if he was not stopped, the whole world would be consumed into chaos and an empire so morally corrupt that the vary values and essence that make us human would cease to exist. Some wars, like WWI, the the past 2 US Wars in Iraq, Vietnam War, the Korean War, and several others were fought for pointless reasons, well at least not at first but then after a while, they started to become pointless. The answer to your question unfortunately will never be "black and white" but always grey, because everyone has different morals, standards, and beliefs. Although what people in the US call the Gulf War, was probably more justifiable, because Saddam was going to take over the region and that would mean total control of the oil prices and much more instability. The second US Iraq war was at first justifiable to kill those responsible for 9/11, however once we killed Al Qaeda and Killed Bin Laden, there was no point in continuing. Oh, and don't get me started on the Cold War, a war that was never actually fought by the way (hence the name COLD war). The Soviet Union under Stalin, was very aggressive and had to end, but once the 1950's there was very little reason for us to logically continue to combat the Soviet Union up until it inevitable collapse in 1991. The Korean War and the Vietnam war were two mini wars that stemmed from the Cold War when Communism was on the rise in Korea and Vietnam, and the US thought that it was the "world police" to try and stop the spread. No country should directly interfere with another country unless it directly involves them.
Wars are caused because gross mistakes are made by those in power. Time passes, and the young are then sent out to fight and die for the mistakes made by the old. Why should the young be forced to do so as these were never their mistakes. The young could not vote or have a say in the matter. If wars need to be waged, let the elderly, politicians and the rich be sent to the front lines first. Once these groups have all died off, then the young can take over. An even better solution would be to lock all the politicians who want to wage a war into a large room and let them fight to the death with clubs. Whichever country's politicians are left standing can be declared the winner.