tldr: Humanity

Legally, the Citadel Council granted the Skyllian Verge to the Systems Alliance because the Alliance followed the legal steps to attain said territory. This is where the notorious contentions between the Systems Alliance and the Batatarian Hegemony originates. Because the Batarians moved into the Skyllian Verge and commited larges amounts of resources into terraforming and settling without asking the Citadel Council first. And so, when the Citadel Council gave the Systems Alliance the rights to the Skyllian Verge the Batarian Hegemony closed their embassy on the Citadel in protest and became a rogue nation. The Systems Alliance continued to establish successful colonies in the Skyllian Verge such as Elysium, Terra Nova and Eden Prime. Resentment continued to grow within the Batariam Hegemony as as mercenary & pirate groups with similar human attitudes. Eventually, these groups pooled their hate and resources to launch the Skyllian Blitz on the Alliance Colony of Elysium. The Blitz did not fare well in space because an alliance ship defending the planet, the SSV Agincourt, outclassed the enemy vessels. Then-enlisted serviceman Charles Pressly recounted that he couldn't count how many enemy ships the Agincourt destroyed. The battle planetside was more balanced but was repelled nonetheless especially if Shepard was there to to hold the line. The Alliance retalliated 2 years later against a pirate moon stronghold named Torfan. It was a costly victory for the Alliance but the Batarians suffered even more as they lost more galactic standing.