The Brothers' War, also known as the Antiquities War, was the legendary great war between the brothers Mishra and Urza that spawned after the two each found half of an ancient Thran powerstone, and wanted both.
The war lasted for decades, devastating Terisiare, before the dramatic conclusion on the verdant island Argoth. There, Urza discovered that his brother had been consumed by Phyrexians, and activated the Golgothian Sylex. The Sylex Blast destroyed the island, activated Urza's latent planeswalker's spark, and initiated the Ice Age.
The impact of this event led to a creation of a calendar called Argivian Reckoning (AR), which places the year 0 as the one in which the brothers were born. It also spawned a general dislike for magic and artifacts, because of their use in the war (e.g., the Church of Tal).