UFO:Alien Invasion

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UFO: Alien Invasion
UFO: Alien Invasion
GenreTurn-based, Real-time hybrid strategy
Latest release2.5
Release dateJune 28th, 2014
PlatformsWindows, Linux, Mac
Code licenseGPL [1]
Media licenseGPL, CC-BY, CC-BY-SA [1]
EngineModified Quake II engine
P. languageC++
UFO: Alien Invasion is a free game. This means that the source code is available to be studied, modified, and distributed. Most projects look for help with testing, documentation, graphics, etc., as well.

Available as a package in:  
Slackware logo.svg Slackware: ufoai 
Archlinux-icon-crystal-64.svg Arch AUR: ufoai-git 
Openlogo-debianV2.svg Debian: ufoai  
Gentoo Linux logo matte.svg Gentoo: games-strategy/ufoai 
UbuntuCoF.svg Ubuntu: ufoai 
OpenSUSE Logo.svg OpenSUSE: ufoai 
Mageia logo small.jpg Mageia: ufoai 
Daemon-phk.svg FreeBSD: ufoai

UFO: Alien Invasion is a hybrid turn-based strategy game and real time strategy game with the objection to defend earth from an alien invasion. The player has to build bases and extend them by building additional buildings, research technologies and develop weapons which are then used to fight the aliens in tactical scenarios.[2]



In the backstory, the world went through a chaotic early 21st Century, which stabilized in the latter part of the century and led to a period of peace and relative prosperity. The game begins in the year 2084[3] shortly after the aliens attacked first Mumbai and then Bonn, Johannesburg, and Bangkok, inflicting heavy military and civilian casualties with little effective resistance.

A mere 18 days after the attacks begin, the player takes command of Phalanx, a hastily formed extraterrestrial response force under the UN which is the resuscitation of a long-defunct program of the USA.

Early Game[edit]

With much of their advanced equipment proving vulnerable to alien stealth, Phalanx initially makes heavy use of ancient, dumber weaponry systems[4] as well as advanced electronic countermeasures and targeting systems to try and reduce the aliens' advantage.

The player starts out with standard ballistic weaponry and combat armor, and must decide how to proceed with research priorities such as obtaining and interrogating live aliens and developing man-portable laser weapons.



The single player campaign is primarily split into two parts which are played alternately.[3] In the Geoscape, the game plays in real time (with optional time acceleration). Aerial dogfights between human aircraft and alien spacecraft take place in Geoscape mode, as well as new facilities and bases being built from there. When aliens are shot down (or land), the player sends a squad of soldiers to secure the crash site. The game then enters the Battlescape mode, a turn based tactical fight against surviving alien forces on site.


Bases and Facilities[edit]

The Geoscape lets a player see all facilities represented on a globe. Bases, external facilites, even such things as UFO crash sites. Additional base construction can be ordered from here and facilities can be built if enough bases exist—each supports only a fixed number of external facilities and there is a global maximum as well.

UFO Detection[edit]

Each Phalanx radar system has two ranges: A smaller detection range within which UFOs will be found, and a larger tracking range, within which already detected UFOs can be followed. This also applies to aircraft.

Aerial Combat[edit]

Dogfighting takes place directly in the Geoscape—there is no separate interception minigame. By clicking on a UFO the player brings up a list of available aircraft and chooses which to send after it. UFOs may also decide of their own accord to hunt down Phalanx aircraft in the vicinity. Once craft are in weapons range, they can begin shooting at each other until one flees out of range or one or both are shot down. A UFO crash site will only result if the craft was downed over dry land and UFOs will sometimes decide to land before they can be intercepted. Landing sites may be approached with a dropship for a mission the same as crash sites, except there will be more surviving aliens to fight, more salvageable equipment and a strictly limited time frame before the UFO takes off and escapes to orbit.

In addition Phalanx are able to install anti-air weaponry at bases and off-base SAM sites. These are able to engage visible UFOs within their range and thereby help with difficult interceptions.

Base Management[edit]


A base exists in a 5x5 grid of underground construction area. The first already contains the most important facilities and a little open space while subsequent bases begin as only an entrance, with additional facilities needing to be built for the base to be useful. With successive bases a part of the grid consists of unbuildable squares.

As base facilities cost a considerable sum to build, have a monthly upkeep, and take some time to build, decisions should be made carefully. It is also advisable to consider defense when building a base as aliens may choose to attack and some facilities will, by the very nature of their function, have entrances accessible from the surface.


If a base has laboratory space and scientists available, research may be conducted there. Research is independent at each base and cannot be combined. In order to research an alien artifact it must typically be available on site, although some projects can be conducted from anywhere. The player simply assigns how many scientists should work on each project and waits.


Storage facilities are required for a base to do nearly anything. They contain nearly anything, from ammunition to aircraft parts or missile batteries. Manmade items can be purchased instantly on the marketplace while exotic items need to be recovered from the battlefield or built in workshops. Simply order production to begin and specify the number of items. Producing human technology in-house does not confer any price advantage but wastes time and workshop space.


From within each base the player can hire and fire employees provided enough living quarters are available for all of them. There are soldiers, pilots, workers, and scientists available for hire. The skill levels of the recruitment pool are random so recruits need to be chosen carefully. A finite number of candidates are available with more becoming available each month. Hired personnel must be paid monthly.

  • Workers have no skill levels. They build new items in workshops. Initially they're of little use but soon the player will need them for disassembling captured craft and constructing newly researched devices which aren't available in the marketplace.
  • Scientists also have no skill levels. They work on research projects, which advance the story and invent new toys for your soldiers to play with.
  • Soldiers have a large set of battlefield skills. These increase as they are used on the battlefield, making veteran soldiers extremely valuable.
  • Pilots have skills for piloting, targeting and evasion. These also increase with use.

Soldiers can have their equipment loadout modified either here or in the dropship loadout. Including those who aren't on the dropship: The option is important if some soldiers are to be equipped for base defense.


The weaponry, ammunition type, armor, and miscellaneous equipment as well as the personnel assignment for aircraft can be edited. New aircraft may be purchased from the marketplace or built in a workshop, if hangar space is available.

There are two basic types of aircraft: Interceptors, which use the small hangar, specialize in aerial combat. Dropships, which use the large hangar, transport troops to mission sites and are generally not very capable in combat.



The Battlescape is a map composed of tiles (or sometimes just one single tile) which are randomly combined to make for a replayable experience. Terrain is not destructible except for very specific scripted conditions (the aliens may break into the Phalanx base through a weak spot in a power plant wall) and windows which may be shot out. Instead the player must rely on other methods of area denial and control, such as creating fires and smoke screens.


The default winning condition in the Battlescape is simply to kill or knock out all opposing forces. Additional winning conditions exist for some maps[5] in the v. 2.6 development branch. For example when the aliens attack a Phalanx base, they can win the mission by occupying the power plant, command center, or antimatter storage facilities for a specified number of turns.

Turn Structure[edit]

The Battlescape is played in alternate turns: Phalanx move first, then the aliens, then civilians. Each unit receives a number of time units per turn and may spend these on actions such as walking or shooting. A player's units may take their actions in any order.

Unused time units may be reserved for reaction fire (somewhat similar to "overwatch" in some games in the genre). In this case, the unit will shoot in the enemy turn if an enemy performs in its sight enough actions to use up the same number of time units as the shot.

One notable exception to the genre is that in UFO:AI it is not possible to save the game during a battle.[6] This decision was made for several reasons. Not only does it complicate[7] development but especially because of the observation[8] that in other turn based games, similar mechanics may lead to higher difficulty[9] settings being developed with the availability of loading taken into account, in such a way that the player is effectively forced to save scum[10]. This is dissatisfying to the players and, as the wiki puts it[11]: "It takes the fear away: and you should be afraid." It is a different and perhaps harsher approach than the optional iron man mode[12] which has been implemented in more recent games in the genre[13].


UFO:AI supports multiplayer matches in the Battlescape, with one player taking the role of Phalanx and the other that of the aliens. Team play is also supported; there can be up to three players on each side[3] and the AI is capable of playing just as it would in a campaign battlescape.


The game has a map editor, called UFORadiant[14].


There is a strategy guide on UFOpaedia.org.



UFO:AI may be considered as a spiritual successor to the 1992 non-free game X-COM: UFO Defense which was originally released under the name UFO: Enemy Unknown[3]. Because of this it may be compared to the UFO: After X series of games from the early 2000's which preceded it, although both took radically different approaches to modernizing the genre. Larger (non-free) commercial games such the Xenonauts series and 2010's XCOM series are younger than UFO:AI and thus obviously did not inspire it.


UFO:AI was created using a heavily modified version of the Quake II engine[3] which was released by id Software under the GPL[15].

Current Status[edit]

Although the last stable release is already 4-6 years old at the time of this writing (the last source release from 2014 and data released in 2016)[16], the game remains in active development[17] and prospective players would be well advised to get a more recent version.


  • Project lead: Martin Gerhardy
  • Lead storyline developer: Ryan A. Span
  • ...and numerous other developers listed on the Contributors[18] page.


External links[edit]


  1. 1.0 1.1 license
  2. Linux Journal article
  3. 3.0 3.1 3.2 3.3 3.4 About page in UFO:AI's wiki
  4. The 1999 downing of an F117 is likely to have inspired this design decision in addition to being arguably more fun gameplay.
  5. Configuring missions for maps in the wiki.
  6. Project lead Martin Gerhardy stated explicitly that saving during battle will not be implemented to prevent save scumming and the decision was made early in the project.
  7. According to story writer Bas Fournier saving the game during battle was deemed not only difficult to implement, but also too difficult for the dev team to maintain.
  8. One player noting the effect of allowing save scumming
  9. TV Tropes, for example, describes the non-free UFO: After X games as "Nintendo hard".
  10. That is, to save the game when getting good results from the random number generator and loading when bad things happen, especially the death of soldiers
  11. Project FAQ
  12. In iron man mode a game exits when saving and saves when exiting, in theory protecting the player from the temptation to load a save file when something goes wrong. There is only one save file for a playthrough and many games will automatically update the save file as soon as something bad happens.
  13. Such as Xenonauts (2014) and XCOM: Enemy Unknown (2012), both non-free games.
  14. Mapping overview in the wiki
  15. Slashdot thread on the GPL release of the Quake II source code
  16. Download page
  17. Commit browser
  18. 18.0 18.1 https://ufoai.org/wiki/Contributors