system
Given the following post, respond as if you're commenting on Reddit:
human
Can someone give me a Halo world overview?

I know the basic story in Combat Evolved, but i have no idea what has happened before it or after.
gpt
Long story short: It's the year 2550 or something and human kind has spread out over the stars, colonizing worlds and fighting some small wars against itself.  The Human Government is the UNSC (United Nations Space Command) and it's HQ'ed on Earth.  The UNSC's largest military base (aside from Earth) is a planet named Reach where they produce ships, train soldiers, etc.

Anyways, at some point, an enemy known as the Covenent make contact with a far off human colony named Harvest (which colloqially was used to grow food for other worlds).  The Covenent invaded Harvest and burned the planet down with powerful weapons.  The USNC sent several ships to investigate, and all but one were destroyed.

Eventually, the UNSC sent a HUUUUUGE fleet (led by Adm. Cole) to attack the enemy, and won!  At the cost of 3 Human ships to 1 Covenent one.  Its was a terrible victory that would set the tone of the Human-Covenent War for like the next what? 50 years?  Anyways, the Covenent are deemed to be such a threat that the locations of places like Earth and Reach are deemed top secret, and human forces are no longer allowed to retreat to them out of fear that the enemy will find them.  This is known as the Cole Protocol.

Anyways, over 50 years, humanity is slowly getting its butt kicked.  While they win some battles, they ultimately are pushed back.  Eventually, the Covenent find Reach and launch a massive assault on the planet.  Hundreds of ships are destroyed, the planet is bombarded, and the human military forces are thrown into disarray.  After Reach falls, all that is really left to effectively resist the enemy is Earth.

This is where Halo:CE starts.  The Pillar of Autumn just left Reach as it collapsed, picking up the Master Chief on the way out.  The enemy chasing them, they couldn't goto Earth or another human planet, so they made a blind jump away from the planet and ended up at Halo.

Unfortunately, the Covenent beat them there, the Pillar of Autumn was shot down, and the fight for the Halo begins.

After the events of Halo, the Master Chief and survivors of the events in the game made their way to Earth.  They did so by hijacking a Covenent Carrier, a huge ship.  They fought their way back to Reach (to find a human ship which might have coordinates to Earth or something), picked up a bunch of other Spartan survivors they managed to find, and used the advanced weapons of their new ship to their advantage.  They eventually figured out how to take Covenent weapons on the ship and advance them to the point where they would kick the crap out of the Covenent ships.  Anyways, they eventually stumble upon the Covenent invasion fleet poised to attack Earth, the Spartans infiltrate the central command ship (a huge space station) and sacrifice their Covenent Carrier to blow the whole fleet and thing up.

The Master Chief made it out (lost everyone else on the huge space station) and took a human ship to Earth.  A few weeks later, he's on a space station getting an award when... badum dum dum: the Covenent attack!

Then Halo 2-3 happens.