The poem "Peace" by Khalil Gibran is a good place to start. The poem is extremely anti-war and features a young woman praying for the safe delivery of her loved one from a war. Later, the young man returns, battered, but safe, and they both praise the coming dawn as a new beginning.
"Your Thought and Mine" speaks powerfully of two radically different viewpoints: one in favor of peace and one supportive of war (and other political ideologies that Gibran finds unpleasant). Here is an excerpt that speaks specifically of war:
"Your thought holds that the glory of the nations is in their heroes. It sings the praises of Rameses, Alexander, Caesar, Hannibal, and Napoleon. But mine claims that the real heroes are Confucius, Lao-Tse, Socrates, Plato, Abi Taleb, El Gazali, Jalal Ed-din-el Roumy, Copernicus, and Pasteur. Your thought sees power in armies, cannons, battleships, submarines, aeroplanes, and poison gas. But mine asserts that power lies in reason, resolution, and truth. No matter how long the tyrant endures, he will be the loser at the end." ("Your Thought and Mine")
Gibran's poetry as a whole is incredibly pro-peace; the poem "Your Thought and Mine" really explores Gibran's stance on religion, war, politics, and society in general.