- Aristophanes - Lysistrata talks about how hard war is on the women at home.
- Homer - The Iliad shows that the Trojan war was pretty much a disaster for all involved.
- Aeschylus - A continuation of the tragedies of the Trojan war. Agamemnon's family was simply shredded in the aftermath.
- Herodotus - The parts of the History we read mainly dwells on the follies of various Persian leaders. Spoilers: they didn't do well.
- Thucydides - The Peloponnesian war was terrible for pretty much every part of Greece.
Some of this is simply timing. Quite a bit of the classic Greek works were written during the Peloponnesian war. It understandably dominated the thinking of the time. But all the same, these are foundational works of Western thought. You could very easily read some of the greats and come away anti-war.
At the very least, this reading has put some caution into my thoughts on the subject. Not that I was rabidly pro-war before. (In fact, the rabid pro-war person is more of a caricature than an actual representation.) But my ideas of the costs and benefits have changed. My threshold for supporting war is much higher than it was before.