1. War is immoral.
Murder is the one crime that we're taught to excuse if it's done on a large enough scale. Morality demands that we not so excuse it. War is nothing other than murder on a large scale.
Over the centuries and decades, death counts in wars have grown dramatically, shifted heavily onto civilians rather than combatants, and been overtaken by injury counts as even greater numbers have been injured but medicine has allowed them to survive.
Deaths are now due primarily to violence rather than to disease, formerly the biggest killer in wars.
Death and injury counts have also shifted very heavily toward one side in each war, rather than being evenly divided between two parties. Those traumatized, rendered homeless, and otherwise damaged far outnumber the injured and the dead.
The idea of a "good war" or a "just war" sounds obscene when one looks honestly at independent reporting on wars.
When we say that war goes back 10,000 years it's not clear that we're talking about a single thing, as opposed to two or more different things going by the same name. Picture a family in Yemen or Pakistan living under a constant buzz produced by a drone overhead. One day their home and everyone in it is shattered by a missile. Were they at war? Where was the battlefield? Where were their weapons? Who declared the war? What was contested in the war? How would it end?
Is it not perhaps the case that we have already ended war and now must end something else as well (a name for it might be: the hunting of humans)?
If we can change our manner of killing foreigners to render it almost unrecognizable, who's to say we can't eliminate the practice altogether?
2. War endangers us.
There are more effective tools than war for protection.
In arming, many factors must be considered: weapon-related accidents, malicious testing on human beings, theft, sales to allies who become enemies, and the distraction from efforts to reduce the causes of terrorism and war must all be taken into account. So, of course, must the tendency to use weapons once you have them. And a nation's stockpiling of weapons for war puts pressure on other nations to do the same. Even a nation that intends to fight only in defense, may understand "defense" to be the ability to retaliate against other nations. This makes it necessary to create the weaponry and strategies for aggressive war. When you put a lot of people to work planning something, when that project is in fact your largest public investment and proudest cause, it can be difficult to keep those people from finding opportunities to execute their plans. Read more.
War making provokes danger.