So I finished Reach. Overall, what a great game. Maybe even better than Halo CE. There was so much variety in the gameplay, such great level design. Also outside of when the game was trying to play sad music because something bad happens, the music kicked ass. I'd rate Halo: Reach a 9/10.
My understanding of the story so far... in Halo Wars they make contact with Convenant, who are looking for Forerunner technology but instead find Flood. The Convenant does eventually find the tech they want but before they can use it, the UNSC blows the planet up. The war rages on for awhile and we get to Reach. The Convenant destroys Reach and slaughter tons of humans and Spartans, but somehow the UNSC has found some Forerunner tech that they think can turn the tide of the war, which they are badly losing. You give Cortana to Keyes to take aboard the Pillar of Autumn and that leads right into Halo: CE.
Reading some Halo lore, it seems I'm missing a ton of into but I assume these are in books or something, and I am not digging that deep lol.
I am a couple missions into CE. It holds up and is still very good.