The developers of Long War, a mod for XCOM: Enemy Within, aren’t messing around. They’ve “removed every bit of code [they] can find that cheats for the player on the lower difficulties.” They’re looking to make XCOM more difficult and the alien invasion more realistic.
Some of the changes they’ve managed to make are incredible, especially considering Firaxis haven’t released any mod tools.
The modders say that while they “loved the vanilla game” they thought they “saw lots of opportunities within the engine for much deeper strategic and tactical play and a greater variety of problems to throw at the player.” Some of those changes include massive bump to alien HP and throwing late game aliens into the first missions you come across. Another is that your home country can be invaded in the first few months of the game, too.
The modders have made it so the aliens have “an active strategy”. They aren’t simply responding to what you’re doing but “gathering resources and conducting research and going on other missions not directly related to XCOM's operations.” This means that you’ll sometimes come across a large alien force in the middle of a big operation while you’re still woefully under equipped to deal with it. To give players an out in this instance the modders have added escape zones to most maps. If things are going south then you can try and retreat.
Another significant change is a new system fatigue. Now, after a mission, “soldiers need to rest for several days to recover from the intensity of combat and exposure to strange alien chemistry.” You can still send them into combat if you must but they will automatically receive a light wound if they get back from the mission. This means they’re then unavailable till they recover.
You can download Long War from Nexus Mods but keep in mind you’ll need Enemy Within to play it. If you've been playing with an earlier version of Long War you won't need to restart your campaign but it is recommended.
The complete feature list can be read on the next page.