UFO: Alien Invasion is a free, open-source game in the tradition of the original UFO: Enemy Unknown. Players must defend earth against an alien invasion by building bases around the world, shooting down alien UFOs and sending in infantry squads to engage the enemies in turn-based tactical combat.
In line with the traditional gameplay of the original UFO: Enemy Unknown, UFO:AI is split primarily between a global view of the earth and short tactical ground missions. In the global view, players will construct and develop bases around the world to track and engage UFOs. Players will have to recruit and equip soldiers, hire scientists to research new technologies, and employ engineers to build new weaponry, aircraft and equipment. Funding comes from nations and the sale of loot acquired from successful ground missions.
In the ground missions, players will take control of their soldiers (8-12) to kill or capture aliens that have landed or been shot down on earth's surface. Technology acquired during these missions will open up new research paths, while research and development will provide new weapons and equipment for soldiers to wield on the battlefield.
The game was initially developed by a small team, which released their final version ("the Christmas release") of the game at the end of 2003. In 2006, one of the project members, Martin Gerhardy, received permission to release the game under an open-source license. The source code and art assets were made available on their Sourceforge page and development resumed under its current form.
- Jan 12, 2008: Version 2.2 released
- Air interceptions
- Random map assemblies
- Jun 17, 2010: Version 2.3 released
- Campaign system
- Rendering engine improvements (normalmaps, bumpmaps, glowmaps, etc)
- UFO crash site missions
- Apr 25, 2012: Version 2.4 released
- Smoke and incendiary weapons
- Jun 28, 2014: 
- Major rebalancing of campaign and combat
Where to Download
UFO: Alien Invasion can be downloaded for free from the project's website.