

See also Installing (OpenXcom) and Mods (OpenXcom).
#Ufo alien invasion vs. openxcom mod
For example, Hybrid Mod (which combines X-COM and TFTD) or X-Com Files require OXCE. They are backward compatible, in general. Any mod may have one of 3 distinct engines as a requirement: pure OXC, "Nightly" OXC or OXCE. This involves much less frustration when you understand basics of mod compatibility. You probably want to use OpenXcom with mods. The code is fully documented and open-source so anyone can take a crack at it.

OpenXcom is an open-source clone of the original X-COM, licensed under the GPL and written in C++ / SDL. The overall gameplay alternates between globe-level, and single-battlefield level, until either your forces are completely wiped out, or you have defeated the enemy invasion. Most, but not all, of these battles will involve an alien ship, either intact or damaged. It is possible to have to flee but still have positive points (considered a marginal victory - you might not have stopped the aliens, but you did enough damage that your funding nations will be happy enough). More accurately, you are scored based on a variety of functions (aliens killed, innocent bystanders killed, etc), and your goal is to be positive. In part 2, you are in a turn-based, tactical, grid-based combat simulation, with fog-of-war your goal is generally to defeat (capture or kill) the aliens in that battle. In part 1, you are focused on globe-level activities: Tracking UFO actions around the world (possibly shooting them down, or sending troops to the sites of ship landings), keeping governments happy with your actions, getting funding money from various governments, and spending it on soldiers and equipment. X-Com is a simulation of an alien invasion of earth.
