I quote Narcogen of Rampancy.net fame here.
In Halo 1, you are (supposedly) the final SPARTAN escaping in an old spacecraft chased by a Covenant armada and crashland on a strange ring, which turns out to be a weapon of unimaginable power and a prison for an unimaginable terror, the Flood, which poses a dire threat not only to you but the entire galaxy. Your marine cohorts are decimated and you're forced to detonate the engines of your ship, killing the Covenant and the Flood and destroying the ancient artifact. To the best of your knowledge, when the game ends, you and Cortana are the sole survivors.
Add the Covenant civil war, the death of the Prophets, Tartarus' betrayal, the Heretics, the attack on Earth, etc.
How much more gritty can it get?