Halo: Reach, from start to finish, is about trying to win while constantly, persistently losing. You don’t just ‘lose’ the planet Reach at the end of the game; you are losing it gradually right from the very start. The game is paced in a particular way so that the player doesn’t feel like they are winning for most of the game before things go wrong, but in a way that the goalposts are constantly shifting, so that the player constantly feels like the situation is deteriorating and becoming increasingly desperate.
To lean on a tired cliché, with every step the player takes forward in Reach, they take two steps back. As the game progresses and the true extent of the Covenant’s presence on the planet becomes apparent to the characters, the goalposts keep shifting. I think of the way the game is paced as almost like walking backwards down a staircase. Each mission begins in a worse scenario than the previous mission hoped to reach. Each mission’s Worst Case Scenario is the next mission’s starting point. No matter how hard you try to make things better in a mission, by the next mission, things will be even worse.
Perhaps the best example is the progress across three missions about halfway through the game. Firstly, in “Tip of the Spear,” the UNSC leads a full-on assault on the secret Covenant army hiding on Reach’s surface, cloaked. As the USNC forced attack this army head on, Noble Six is sent off to take out several anti-air gun batteries so that the UNSC battle cruisers and their bigger guns can move in. Once this is achieved, there are more cloaking stations to take out that are hiding something.
At the end of this mission, the player takes out the tower’s shields, and a giant UNSC cruiser drifts in to finish it off with an explosive MAC round. It does this, but seconds later, a beam of plasma bursts down from the sky and rips the UNSC cruiser in half. What was being cloaked by the towers, it seems, is a Covenant super-carrier, a massive Covenant ship just floating above the battlefield.
As the level progresses, the player takes a step forward; as it ends, they take two steps back. It started with the knowledge there is a giant Covenant army on Reach, but one that can be dealt with. The entire mission is spent dealing with that army, and the player takes their step forward as they take out gun batteries and the UNSC forces make ground. But then, as the super-carrier appears at the end, two backward steps are taken, and the next mission starts in a worse place than this mission began.