First off, if anyone who reads this has fought or lost a loved in war, I would first want to thank that person for their service and second I would like to send my condolences. The following paragraphs deal with my personal thoughts when it comes to war.
I tend to group the reasons for war over two things: culture and resources. Out of those two I find that the war over resources is the only just kind of war. Why? Because in a war over resources, people are killing each other over physical things and the losers of said war will typically be allowed to survive if they concede some of the resources that the victors were looking for. A prime example of this would be the Roman strategy to conquer the land and then remain lax with the conquered peoples (unless they rebelled or refused occupation) and slowly, but surely the locals would accept Roman occupation and bring revenue and trade to the region. Now of course there have been deviations from this type of behavior, but that usually has to do with the cultural aspects which I am about to bring up.
War fought over differences in culture is a different matter than war fought over resources because when the victor wins, it usually spells doom for the defeated. Total defeat results in usually the total destruction of the vanquished where their entire civilization is razed and assimilated by the conqueror. Mass genocide results to any of those who still believe in their gods and not the "right" gods of the winning faction. Temples are sacked and priceless gilded relics are melted into golden bars to be sent back to corpulent monarchs as they revel in obscene palaces built by death and suffering.
Sadly, it seems that Christianity is the guiltiest cultural entity to commit such terrible acts that are against the teachings of Jesus of Nazareth. The Crusades, the Reconquista, the 30 years war, and the French Wars of religion were caused by Christian leaders who wanted to rally forces for the sole purpose of power, but wanted to decieve their people into a frenzy to kill those who are different. Then it seems that there is no such thing as a cultural war, but a resource war that wants to find a higher excuse to invade someone to get their riches. It seems that recently there is a cultural war occurring between the Jews and Muslims over ownership of the holy land. Now it may seem like this is war over culture, but in reality these Muslim terrorists are only using their religion as an excuse to try and take their land back.
Now with that out of the way, I would like to discuss about how the romanticism and glory of war is all but extinct. The last romantic warrior died at the end of WWII in a car crash, his name: General George S Patton, who would be immortalized in the film Patton. He was born in the late 1800s and was apart of a dying breed of romantics of war. By his duty in WWI as the first American tank commander, war had changed drastically since the days of Homer. No longer did grand armies fight face to face in melee combat, they stood far away with machine guns and mowed down the waves of attackers who charged over the top to run through no mans land and take the opposing trench. This led to not thousands, but millions of causalities by machine gun fire and the new deadly weapons of modern warfare. War was now a mechanized game of Russian Roulette and it would continue that way into WWII.
By the end of WWII, we as a species developed the very doomsday device that could wipe us off with the flip of switch, the atomic bomb. Now wars did not have to be fought on the battlefield, but through conversations in nuke silos around the country.
Now don't get me wrong, war brings out the very worst in people, but it has its pros. For one, we English speaking have a debt to pay for war since it was the William the Norman who conquered England and the mixture of the French speaking Normans and the Anglo-Saxons brought upon the birth of modern English! But then again French is only a product of the peoples who spoke Latin, which was brought upon during the conquests of one Julius Caesar.
War also brings the best in people, in almost every war, there are stories of men who fought and died bravely for their country people and their fellow comrades. The 300 Spartans and 1,000 Thespians come to mind when they fought and died on the third day of the Battle of Thermopylae. Another would be the deeds of Audie Murphy who became one of the most decorated soldiers in history.
So in conclusion I would like to say that war has a place in human society, no matter how advanced we.