After discovering the Charon Relay, humans began a rapid expansion thanks to their new-found connection to the mass relay network. Hoping to expand their territory and driven by immense curiosity about the galaxy, they began activating every mass relay they could find.
However, in 2157, their actions caught the notice of the turians, who found human explorers reactivating an inactive mass relay known as Relay 314. This was forbidden under the Citadel regulations after the Rachni Wars, but instead of negotiating, the turians opened fire. One starship escaped to warn the Alliance; a retaliatory force destroyed the turian vessels, and the situation quickly escalated into war.
To the turians, it was merely a police action against an ignorant species violating Council law; to humanity, it was their first encounter with another intelligent race - and it would shape their views of the galaxy for decades to follow.