The many species of the Milky Way were almost destroyed by the Halo Array nearly 100,000 years before the Humans truly rose to power. Prior to this, Humans has a sizeable Empire but were constantly at war with the Forerunners, forcing some to flee to the Trek Galaxy - an act which would ulimately save them and allow their society to continue.
The Milky Way Galaxy has a vast and diverse history, and is thought to be the largest and most powerful of the Galaxies in the League, as it leads the 'big three' of Galaxies in the League, the other two being the Trek Galaxy and the Force Galaxy. The Milky Way Galaxy has seen Empires rise and fall, species evolve and die, and numerous invasions from extra-galactic forces such as the Reapers, the Flood and the Galactic Empire.
Despite the idiocy of the Forerunners, this Galaxy has continued to be the capital of the League and has supported several other Galaxies in the process, most notably the Boe Galaxy and the Mushroom Galaxy. The Economy of the Milky Way is stable, despite the best efforts of the Quad, and its military is an effective extension of the League's combined military forces as it is led by Jorgey himself.