Firefights in shooters are usually defined by positioning more than anything. Where your Zombiemen and Imps are pretty much defines how the whole thing is gonna go down, and just pulling out the right weapon for the job is pretty much the big decision the player makes when engaging. Halo is different. Positioning matters, but it's not always due to the designers hand that the player and enemies end up in their positions - it's the aliens themselves choosing where they go. Reloading a save for the same encounter will still often see it going drastically differently each time, and this is due to a multitude of factors: Master Chief has a limited arsenal at all times, and the enemies you fight directly tie into this. You will actually need to consider which weapon you want to keep as you can't predict which enemies will be ahead of you, or what they'll be equipped with, or where they'll be. Master Chief also has a shield system that will work alongside your healthbar, with which you get a brief window of protection from oncoming strikes but not full-blown protection; giving you just enough time to perform any risky maneuvers you may need. Everything you do is slightly delayed; from jumps to grenade throws and explosions to reloads delaying your fire to punches landing, giving you a constant need to think about every action you take. The real icing on the cake here though is the intelligence of the Covenant enemies, and how they interact with everything else.

The Covenant has, by no stretch of the imagination, probably the best artificial intelligence ever seen in the genre. The designers have claimed they merely attempted to make them seem smart rather than be smart since they weren't sure how to do that, but I'd argue they hit two birds with one stone. The real trick is that they're reactive, and equal. Reactive in the sense of emotionally expressing reactions to almost every situation, but reactive also in the sense of seemingly playing alongside the player for each gunfight. They'll flank you, toss grenades whenever you get into a comfy position too long or just overwhelm you with numbers. For almost every strategy you have, the Covenant have something up their sleeves to counter it usually. Each Elite has a shield system of their own, so every single fight you get into with an Elite will likely lead in you both taking cover to regenerate since Halo knows to also give the foes some self-preservation instinct. Want to charge in and just damage the Elite? They'll probably do the same; get pissed off and charge at you just hoping to kill you before you put them down. Hell, you might try sniping an Elite only for him to hop in a Banshee and start circling you in the air, and the thing is; this is only the Elites, and while yes: they are the most intelligent Covenant enemies, the true magic is emergence. This is only describing one Elite, what about two? What about his squad of Grunts? Well if you leave the Elite alive they'll have the confidence to charge in and try attacking you on their own terms, but if you take out the Elite first you'll be opening yourself up to them and the Jackals fire. Your assault rifle will make short work of the Grunts, but can it counter the Jackals and Elites shields? That's something new to consider, so you'll want to keep multiple types of weapons on you at once for this situation. What if there's too many to take out? Well there might be a Warthog nearby which you could straight up crash into them with, or just have a Marine fire for you as you strafe around them. Every single encounter requires you to rethink and preplan how you'll handle things, and you'll always need to keep on top of ammo/charge per weapon too; so you can't rely on, well, old reliable, forever.

Just when you've gotten comfy, the game throws the Flood at you. As divisive as they are, and as arguable the Library's quality is, I'd wager they're a necessity. They're a great shake-up to the more strategic combat centered around the Covenant, requiring you to pretty much treat every encounter with them as a gauntlet as they eat up bullets and plasma but just don't go down, and will revive other enemies as more of themselves. But just as is the case with the Covenant, emergence is their true strength. Late-game Halo: Combat Evolved has you overseeing armies of Flood and Covenant fighting to the death and it's your job to just get by while the games systems play their own little RTS as you go off shotgunning more zombies.

The truth to Halo's design is it's multifacted, and it makes the most out of very little. Compared to your average shooter there's not much variety in the enemies or weapons but the core behaviors are so nuanced and dynamic that they change moment-to-moment, encounter-to-encounter. It gives you a sandbox of weapons and vehicles against decently tough enemies (though this will vary depending on the difficulty you pick, but I recommend Heroic as it seems to be the intended experience) and asks how you will deal with it. Halo gives you tons of ways to play, but the enemies can play at that game too and utilize basically anything you can. It leads to an immense creativity in encounters that comes from the most fun form of problem-solving that makes it, in my eyes, one of the crowning jewels of the genre. None of the sequels rivaled the quality of the combat, because it's already the pinnacle of evolution.