system
Given the following post, respond as if you're commenting on Reddit:
human
[Halo] So can anyone give me a timeline from 2015 to the year 2552?
gpt
By 2080 we had a developed and robust space program and had colonized Mars to a large degree. About 80 years later, in 2164, the first major rebellions on Mars began, after the second and third generation colonists became disillusioned, which would lead to the formation of the United Nations Space Command, or UNSC, to counter them. While the fighting is rather intense, the rebellion is almost completely subdued. In 2291 we develop the Shaw-Fujikawa Slipspace Drive, an FTL technology, allowing Humanity to reach out into the stars. Over the next 200-ish years we colonize over 800 worlds.

In 2517 the ultra-secret SPARTAN II program starts, largely in response to a resurgence of insurrectionist activity, and a six-year-old John begins his journey to become Master Chief. Less than 8 years later, in 2525, we would encounter the Covenant. Relations become violent almost immediately, and the newly created SPARTAN IIs are mobilized against them. 

In 2552 Reach is invaded, and the Human-Covenant War swings heavily out of our favor.