War is wrong. Forget the morality stuff. Forget love thy neighbour, Forget forgiving. Forget any morality preached by any religion in the world. Forget all that.
War is wrong because it is no longer practical. It's broken; and it won't work any more. That's why it's wrong.
One of the reasons it's wrong is the one I wrote of earlier, when I pointed out how easy and cheap it once was for western powers to conquer vast areas, to make huge profits from the killing. But that stopped working at least as far back as the 1940s - and maybe we can take it back even to the Russo-Japanese War of more than a century ago. We can't win wars as we used to. The rest of the world has caught on, and has figured out how to beat us even with primitive resources. That's one reason war is wrong.
The other reason is that it has become so destructive of life and the environment that we can't survive it. The great turning point was perhaps the American Civil War of the 1860s. By that time, the development of explosive shells, along with the development of repeating weapons, made war hugely expensive in human life. To that, one can add the use of the airplane as a bomber - and a very indiscriminate bomber.
High explosive and aerial bombing, together, made wars, at least as early as World War One more dangerous to civilians than to soldiers. By now, it is not unusual for civilian deaths to outnumber military deaths a hundred to one. Indeed, even the most conservative estimates would place the civilian to military casualty rates for Vietnam and Iraq far higher than that.
For the last 65 years, we have also had nuclear weapons. Will they be used? Of course they will. If we keep warring, someone will resort to the use of nuclear weapons. It has already been quite openly threatened by leading countries of both sides. Indeed, the decline of the West's ability to win easy wars in the old way makes it almost certain that the West will be the first to resort to nuclear weapons.
We can add to to that the largely unstudied damage to all of us and to Earth itself by the increasing use of chemical weapons and nuclear waste. We cannot survive that, even if the increase levels off.
We have to go to war to fight terrorism? That's nonsense. First, fighting terrorism is the major cause of terrorism. Second, we in the West are the world's largest users of terrorism. Just take a count of which groups have suffered the heaviest losses of civilians over the last 60 years. Hint: none of them are groups of European or North American origin.
We could always wiggle with religious values so that Thou Shalt Not Kill could mean Thou Shalt Not Kill (unless the oter side is bad.) But it is not possible to wiggle with the facts of explosive and chemical power that is indiscrminate in its killing, and its poisoning.
War doesn't work anymore. That's why it's wrong.