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The Cylons quickly establish the initiative and used overwhelming force in both symmetric and asymmetric methods of operation to effect the total devastation of the Twelve Colonies and to virtually succeed in annihilating of the human race.
Sesha Abinell gathered information on Cylon modus operandi as part of her ill-fated plan to exact revenge for her husband's death and expose military coverups and collusion with the Cylons. Her computer monitor in "Sacrifice" details several tactics of the Cylons from her point of view:
Shown in the episode, "33"
The Cylons are machines that do not require food or rest. Combined with the work of Cylon agents that have infiltrated the members of the Fleet, the Cylons can terrorize and force the Fleet to tax and drain their limited resources. The Colonial military support is also eroded through exhaustion as well as wear-and-tear. The driving tactic of ambushing the Fleet every 33 minutes for 5 consecutive days greatly limited Galactica's ability to repair fighters as their hangar deck crews as well as command crews were pushed to exhaustion and inefficiency.
The humanoid Cylons are mostly organic, and thus require food and rest by definition; however, they do not require nearly as much as normal humans do, as demonstrated by the Valerii Cylons in "33" and "Tigh Me Up, Tigh Me Down".
Assault on Natural Resources
Cylon agents pose the most fundamental threat to the security of the Fleet and to the remaining survivors of the Twelve Colonies. Because humanoid Cylons are visually indistinguishable from true humans, they form an asymmetric threat to the densely packed and defenseless civilian elements of the Fleet. Destruction or control of the Fleet's limited resources such as water supplies or tylium are of high importance.
Humanoid Cylons, be they "sleeper agents" such as Boomer or self-aware persons such as the first Aaron Doral copy (Miniseries) possess strong emotions that are indiscernible from the emotions of a human. As agent copies are archetypes of human personalities, each humanoid Cylon is specialized to affect the humans they contact in particular ways. A good example of this manipulation is when Boomer, no longer a sleeper agent but still an enemy manipulates Galen Tyrol into helping her using his old feelings for her. However, Boomer admits before she leaves that she hadn't been lying to him about what she felt when she was manipulating him. For more on humanoid Cylon personalities, see the humanoid Cylon article.
Shown in "Litmus"
As the personality of a humanoid Cylon can be resurrected under the right conditions, and as such Cylon agents may intentionally expose themselves to destroy a Colonial resource. A second copy of Aaron Doral detonates himself inside a corridor in Galactica, nearly killing its commanding officers.
After the destruction of the Resurrection Ship, the Cylons adjusted their tactics from direct assaults to hit-and-run attacks, to avoid permanently losing great numbers of Raiders (or humanoid copies aboard Cylon vessels) in battle.
Cylon impregnation and reproduction
As per their God's commandments, the Cylons have used captured Colonial refugees still alive in the remains of the Colonial cities in an attempt to reproduce themselves with human hosts. In cases where a female human refuses to cooperate, the Cylons attempt to artificially fertilize her eggs with humanoid Cylon genetic material. In cases where the woman collaborates, the normal method of intercourse is tried. In other facilities, the Cylons attempt to fertilize female humanoid Cylons with the genetic material of human males. The first successful fertilization and birth of a Cylon-human hybrid was accomplished as part of an experiment in which Lieutenant Karl Agathon was set up to fall in love with an Eight whose true nature he was unaware of.
Such tactics are demoralizing to Colonials who believe strongly that Cylons are purely machines; they may feel that such tactics are a form of rape or perversity that is intolerable, as expressed by Kara Thrace to Samuel Anders (Lay Down Your Burdens, Part II). Conversely, human Colonial officers on the Battlestar Pegasus committed gang-rape of the humanoid Cylon known as Gina Inviere.
There are twelve models of humanoid Cylons, each with a personality archetype considered one of 12 human behaviors or traits. The number of copies of a particular model are unknown, but believed to be very large, further complicating Colonial identification of one model and their actions versus another copy. Seven models are eventually revealed to have untold numbers while the last five, the Final Five have one unique copy that posseses Cylon abilities, but lack knowledge of their true nature and are on the human side.
Sleeper agents within the Fleet
Sleeper agents are not aware of their true nature. A low-level personality overrides the human personality as required to perform espionage or sabotage. Boomer is a sleeper agent that rebels against her suspected nature but eventually succumbs to it when her sleeper personality directs her to shoot Commander William Adama at point-blank range in an assassination attempt (Kobol's Last Gleaming, Part II). Boomer is the only known sleeper agent in the series, although there may have been more on other ships before the attacks. Afterwards, when Boomer downloads to a new body, she has trouble accepting her true nature, but eventually does and becomes a serious threat without being controlled by any programing.
Cylon Agents in the Fleet and key positions before the Attack
Shown in many episodes and "The Plan"
As part of their plan, the Cylons placed agent in strategic positions to do the most damage: for example one Number Two was posted as the Defense Minister's yoga instructor and a Number Six, later named Caprica-Six was sent to infiltrate the Defense Mainframe to shut it down when they attacked with the help of Gaius Baltar. After the attacks, several Cylon agents were in the fleet: two Number Sixes, one named Shelly Godfrey, the other refered to as Tough Six, one Number Two, a Number Eight known as Boomer who was a sleeper agent, a Number Five, a Number Three and a Number Four. These agents acted in various roles in the Fleet, commiting various acts of sabatoge, but mostly failed with all being killed with Tough Six's fate being the only one unknown. The Cylons also managed to get agent Gina Inviere on board the battlestar Pegasus where she performed acts of sabatoge before being exposed during the Battle of the Communications Relay when Kendra Shaw spotted another Six leading a boarding party of Centurions.
Takeover by Brute Force
The symmetric threat posed by the Cylons resides chiefly in the large, well-supplied, technologically sophisticated and well-coordinated space-based naval forces. These are comprised of space-superiority fighters, troop carriers, large capital warships, associated support vessels, and a Resurrection Ship, which is responsible for the recovery of killed humanoid Cylon and Raider personalities. An individual baseship appears to be less than a match for a Mercury class battlestar in combat, but there are presently far more of the former than the latter. It appears that while baseships and battlestars are designed to be carrier/battleship hybrids, baseships lean more towards a carrier as shown by their proportionately larger fighter capacity, while battlestars are better at direct engagement between capital ships. Normally if a baseship faces a battlestar with its full fighter compliment, it poses a serious threat even to Mercury-class battlestars. However, if a base ship's Raider wings are lured away (as in the Battle of the Resurrection Ship) and it is then engaged directly by a battlestar's primary gun batteries, the basestar would stand at a great disadvantage.
The superiority of numbers of the Cylons, be it ships or troop units, leaves the Colonial remnant with very limited defensive options and virtually no offensive options should they encounter Cylon forces. Tactics seen include:
After "Downloaded" and the New Caprica Occupation
The Cylon society's direction changes temporarily after two Cylons convince the collective that the human genocide was wrong ("Downloaded", "Lay Down Your Burdens, Part II"). The change is caused by two key Cylons: Caprica Six, who was in great part responsible for the destruction of the Colonial Fleet; and the resurrected Boomer formerly on Galactica. As a result, the Cylons appear to have form a new tactic in their occupation of the survivors of the human race on New Caprica : The role of the benevolent dictator. However, humans begin to resist the occupation in increasing numbers (Occupation) and eventually escape Cylon control (Exodus, Part II).
With New Caprica abandoned, the Cylons drop their goal of quasi-benevolent co-existence, returning to space to pursue the Fleet. They mix military and diplomatic tactics, deferring an attack in one instance to confer with the Colonials above the algae planet regarding the Eye of Jupiter. The Cylons now concentrate on finding Earth itself, racing with the Colonials for clues to its whereabouts (The Eye of Jupiter).
Secretly inserting an agent for a secret mission
After the destruction of the Resurrection Hub in the Battle of the Resurrection Hub in the Cylon Civil War, Cavil seems to abandon the pursuit of the Fleet for a time, although he apparently knows where it is given the fact that Boomer was able to find it. The Final Five are the only ones who know the secret of resurrection and as all he has is the resurrected Ellen Tigh, Cavil devises a plan where he has Boomer "help" Ellen escape vivesection to find out the secrets of resurrection when in reality her true purpose is to get on Galactica and kidnap Hera Agathon so the Cylons can learn how she came about. Boomer succeds in kidnapping Hera, but bonds with the girl and regrets her action while at the same time drawing the anger of the Colonials and Rebel Cylons onto Cavil, leading to the Battle of the Colony where Galactica destroys the Cylons.