Operation Guerrero was a short conflict between the Federated Commonwealth, the Free Worlds League, and the Capellan Confederation. Despite lasting only four months, the conflict set the stage for serious repercussions over the next two decades, ultimately precipitating the FedCom Civil War. The conflict was from September to December of 3057.
After the death of Joshua Marik in 3057, Victor Steiner-Davion replaced the child with a double to keep war supplies from the Free Worlds League coming to his forces. Thomas Marik discovered this substitution in September of that year, and ordered the Free Worlds League Military to attack the Federated Commonwealth in retaliation for the deception. Most of these worlds were former Free Worlds League planets that the Federated Commonwealth had captured and integrated into the Sarna March. Sun-Tzu Liao ordered the Capellan Confederation Armed Forces to attack Sarna March worlds as well to support his (possible) father-in-law.
The Federated Commonwealth was completely taken by surprise by the attacks, as most of their attention was on the Clan front. The LIC, DMI, and MIIO erroneously attributed the movements of the FWLM attackers along the Sarna March border to routine troop movements.
FWLM First Wave
In September 3057, the FWLM attacked the Callison, Denebola, Marcus, Van Diemen IV, Talitha and Wasat systems. These six systems were each attacked by three FWLM regiments. The systems were chosen specifically because they were garrisoned by Federated Commonwealth troops.
Callison was attacked by the 4th Free Worlds Legionnaires, 1st Free Worlds Guards, and Silver Hawk Irregular's Falcon regiment. The 11th Lyran Guards withdrew in good order from the system, basically leaving the system to the FWLM forces. The remaining planetary militia resisted for several days.
Denebola was taken by the 6th Free Worlds Legionnaires, Atrean Hussars, and 4th Marik Militia. Though normally defended by a Sarna March Militia unit, the Denebola FTM was conducting exercises on Caph with Gustafson's Grenadiers which left the system's defense to the small planetary militia. The planetary militia surrendered without firing a shot.
The attack on Marcus was very different from the Callison and Denebola attacks. Defended by the 12th FedCom RCT, Marcus was attacked by the 7th Free Worlds Legionnaires, 5th Marik Militia, and 12th Atrean Dragoons. The FedCom 'Mech regiment attacked the larger force in a suicidal attempt to provide time for the rest of the FedCom defenders to withdraw. This attack destroyed a third of the Seventh Legionnaires, but the Fifth Militia and Twelfth Dragoons circled behind the Twelfth FedCom's 'Mechs and destroyed them.
On Van Diemen IV the FedCom defenders successfully held off the 10th Marik Militia and 23rd Marik Militia regiments, and the 3rd Oriente Hussars until December, when the 1st Sirian Lancers reinforced the FLWM.
FWLM Second Wave
With more time to prepare, the FedCom defenders did marginally better at defending their systems from the Free Worlds League forces that attacked, though they lost Devil's Rock, Oliver, Castor, Pollux, and Zosma. Unlike the first wave, the FWLM didn't always use overwhelming force when taking these systems, allowing the FedCom defenders to give a better performance.
On Devil's Rock the 3rd Free Worlds Guards were rapidly bogged down in a harsh fight with the planetary militia, but eventually crushed all organized resistance.
Pollux's defenders were limited to two regiments: One each of armor and infantry. The other five regiments either withdrew to the Lyran Alliance or joined the Free Worlds League, so the 34th Marik Militia gained control of the system within a week of their arrival.
Role of the Capellan Confederation
Though the death of Joshua Marik and the attempt to deceive the Captain-General was the excuse given for starting Operation Guerrero, the Free Worlds League Military wasn't the major aggressor in the struggle. Once the FWLM forces took possession of the eleven Sarna March systems that had belonged to the Free Worlds League prior to the Fourth Succession War, Thomas Marik ordered his troops to consolidate their holdings.
Sun-Tzu Liao, however, had been fomenting rebellion on many Sarna March worlds since he took the Celestial Throne in 3052 and wouldn't be satisfied until all the former Capellan worlds were once again part of the Confederation. While engaged to Isis Marik he also pushed Thomas to assist him in attacking the Sarna March. For five years Thomas refused, but when Operation Guerrero began, the LCCC assigned the tactical and strategic command rights of several mercenary units to the CCAF.
Capellan Preparations Pre-Guerrero
The Capellan Confederation sponsored pro-Capellan terrorist groups on former Sarna March worlds, and in 3056 these groups stepped up their attacks on Federated Commonwealth forces. The most well known terrorist group, Zhanzeng de Guang (War of Light), struck police and militia units on Aldebaran, Gan Singh, Zurich, and Styk. Throughout 3056 and 3057 the Zhanzeng de Guang and other terror groups set up cells on Acamar, Fletcher, and Nanking. These groups did whatever they could to destabilize the Davion-installed governments, but neither the CCAF or FWLM would commit troops to support their efforts until Operation Guerrero was authorized.
CCAF First Wave
Five Warrior House regiments attacked Liao on the same day that the Free Worlds League announced hostilities with the Federated Commonwealth. The Liao SMM surrendered without a shot being fired and the system fell immediately. At the same time, the Zhanzeng de Guang took control of Zurich, overthrowing the existing pro-Davion government.
In the invasion zone bordering the Confederation proper, Sun-Tzu Liao launched invasions of nine worlds. Regiments were used to confront lone Davion battalions, and trios of regiments to attack single AFFC regiments. All nine invasion were complete successes. Simultaneously with this, revolutionary activity on eight nearby worlds was stepped up to the point the pro-Liao forces were engaged in hit-and-run attacks that would seriously fatigue the AFFC garrisons, leaving them vulnerable for the second CCAF wave. 
CCAF Second Wave
At the start of Operation Guerreo's second wave, Sun-Tzu Liao activated guerrilla networks on Caph, Keid, New Home, Epsilon Indi, and Northwind in an attempt to return the worlds to the Capellan Confederation. These revolts were successful on each world except Epsilon Indi, and the FWLM sent mercenary units to every world except Keid. (Keid would later join the Lyran Alliance.)
On Northwind, home of the famed Northwind Highlander regiments, the pro-Liao terrorist activity prompted Victor Davion to send the 3rd Royal Guards to insure the planet didn't leave the Federated Commonwealth. The Third's commander attempted to take the planet by force when on-world Highlanders started debating who they should fight for. This action prompted the off-world Northwind Highlander units to return home to liberate their planet, practically destroying the Third Royal Guard RCT in the process. Sun-Tzu declared Northwind an independent Confederation barony and permanently ceded it to the Highlanders.
The Black Cobras were assigned to secure Nanking for the Capellan Confederation, which would bring the system's factories under CCAF control. As the Cobras were making planetfall, their DropShips were destroyed by explosives placed by saboteurs. This effectively wiped out the Black Cobras as a whole, and Smithson's Chinese Bandits (a Marik mercenary unit already on Nanking) surrendered to the planet's defenders when a Federated Commonwealth relief force arrived.
There were several important outcomes that resulted from Operation Guerrero's success. The most obvious was the destruction of the Federated Commonwealth when Katherine Steiner-Davion formed the Lyran Alliance. Her actions, and Victor's efforts to oppose her, ultimately led to the FedCom Civil War.
Though the Free Worlds League gained eleven systems it had lost, the real victor was the Capellan Confederation. Most of the Capellan Confederation's Sarna Commonality worlds, controlled by the Federated Commonwealth since the Fourth Succession War, would return to the Capellan Confederation. (Ironically, this didn't include Sarna, which became one of several independent states in the Chaos March.) Sun-Tzu Liao would also create a new commonality centered on Liao.
The other major outcome of this conflict was the creation of the Chaos March, where forces loyal to the Capellan Confederation, Free Worlds League, Federated Suns, and Word of Blake struggled for control of many of the former Sarna March worlds. Rather than a single strong entity that the Capellan Confederation couldn't defeat, the Chaos March was populated by at least six minor and short lived independent nations. Sun-Tzu Liao's Xin Sheng-inspired CCAF would have little trouble conquering these nations in the next few decades.
- Era Report: 3062, p. 23, "Timeline of Events"
- Shattered Sphere, p. 74, "Operation Guerrero, First Wave"
- Field Manual: Lyran Alliance, p. 114
- Shattered Sphere, p. 74 (Note: Third FWLM regiment and FedCom defending unit not named in source.)
- Shattered Sphere, p. 68
- Shattered Sphere, p. 68
- Bred for War, p. 100
- Field Manual: Lyran Alliance, p. 98
- Shattered Sphere, p. 68 "The Highlander Gambit"
- These events are detailed in the novel Highlander Gambit by Blaine Lee Pardoe.