Though during the 1940's the existence of mutants was unknown to the general public, a few individuals were aware of the coming of Homo Superior, and that they had the potential to replace baseline humans as the dominant species of Earth. Thus Weapon Plus was created to address the so-called mutant problem. What was ignored by everyone involved in Weapon Plus is that its mastermind, John Sublime, was actually the host body for a sentient bacteria present on every living creature on the planet, save for mutants, who were genetically immune to the Sublime infection.
The first nine installments of Weapon Plus were partially successful. Weapon X produced a number of agents, though it branched off and even became opposed to Weapon Plus's interests. To prevent subsidiary programs from going rogue, Weapon Plus directly oversaw the creation of the last living weapons operating in the clandestine facilities of The World, employing Bolivar Trask's Sentinel technology.
Throughout the decades, Weapon Plus has used increasingly more extreme methods to create its supersoldiers. Captain America was enhanced to peak human levels. Weapon X employed genetic alteration, brainwashing and memory implants. The latest creations were bred specifically to become mutant-hunting weapons much deadlier than Sentinels.
In the non-canonical novel "Wolverine: Violent Tendencies", Weapon 0 is referred to as "Weapon Null" and is an early and dismissed branch of a super-soldier program completely different to the Weapon Plus program of the comics. Weapon Null was focused on the utilisation of bioenhancement to upgrade soldiers into organic self-contained weapons; the grotesque creations require extensive and expensive maintenance and transportation conditions, so the program was shelved. During the course of the novel, Wolverine encounters four of Weapon Null's more successful creations: "Slammer", "Blowtorch", "Cypher" and "Bipolar".
Two other subjects, Clinton McIntyre, a.k.a. Protocide, a failed experiment who was placed in suspended animation and was revived in the modern era by AIM, and the first mutant experiment designated Queen seem to have occurred prior to the Super Soldier Serum being tested on Steve Rogers.
Reinstein's early attempts to refine the formula resulted in African American super soldiers (most prominently Isaiah Bradley). Three hundred African-American soldiers were taken from Camp Cathcart and subjected to potentially fatal experiments at an undisclosed location, as seen in Truth: Red, White & Black in an attempt to recreate the Super Soldier formula. Only five men survived the original trials; hundreds of test subjects left behind at Camp Cathcart and the camp's commander were executed by US soldiers in the name of secrecy, the families of the three hundred were told that they had died in battle. Isaiah Bradley was the lone survivor.
Although there were many later attempts to recreate or reverse-engineer Project: Rebirth's Super-Soldier Serum, none are known to have been involved with Weapon Plus except for the attempt that resulted in the creation of Isaiah Bradley's son Josiah X. Weapon Plus considers Captain America as its most successful creation, despite the fact that Rogers has been at odds with the United States government a number of times. It should be noted that Project: Rebirth was retroactively made a part of the Weapon Plus after WWII when Weapon Plus was actually formed.
The United Kingdom also had its own version of Project: Homegrown, aka the Black Budget, which managed to create the team known as the Super Soldiers: Dauntless, Gog, Dreadnaught, Revenge, Victory, Invincible, Challenger, and some unnamed super-soldiers.
The Mercy Corporation, an off-shoot of S.H.I.E.L.D. that worked on super-soldiers and eventually broke off, also had its own unit of super-soldiers, using a serum similar to previous derivatives of the Super-Soldier Serum from Weapon I. Their agents included Jack Reno, Keel, Kyle, Agent Villarosa, Agent Davis and Agent Milo.
In Wolverine: The End, by Paul Jenkins, it was revealed that Weapon IX was, in fact, Wolverine's older brother John Howlett (first introduced in Origin), long thought deceased. However, due to the inherent instability in the possible futures depicted in Marvel comics, it is unknown if this is regarded as canon, as the storyline depicted in The End is not considered canon.
Weapon X, tenth in the series, is the X-Man Wolverine, and this project would branch off into its own series of subjects. The trademark of Weapon X was the use of mutants as test subjects and the employment of memory implants.
Weapon Plus created Weapon XIII Fantomex, whose real name is Charlie Cluster 7, also at The World. However, Fantomex rebelled against his creators. Like in the case of Weapon XII, Fantomex's powers derive from Nanosentinel technology.
The human DNA of the inhabitants' ancestors in this artificial environment was spliced with sentinel microtechnology, meaning they are no longer humans in the traditional meaning of the word, as they have evolved into a race of mutated and naturally born cyborgs refined through eugenics.
Because the scientists have absolute control over the time inside The World (according to Fantomex the time is "liquid"), both freezing it and speed it up so that decades and centuries can pass within it while time runs normally in the real world, this opens up for new possibilities within the areas of eugenics and genetic engineering. The most promising individuals in each generation are selected while the others are terminated. Also natural selection in the form of the survival of the fittest is possible by exposing the population to different forms of selective pressure. This way, new breeds of superhumans can be evolved within a few months, instead of hundreds or thousands of years. Half a million years inside The World represents only eighteen months on the outside.
The World's facilities contains a population (with its own religion, history and culture) that is led to believe that beyond the World's limits there's nothing else than endless rock and that mutants are coming to destroy them. Like in Logan's Run, those who are selected for termination believe there is a reward awaiting them at the other side. The World is also filled with numerous experiments and prototypes of Weapon Plus, such as car-cops.
The World was partially destroyed when agents of AIM attacked the facilities in order to recover the technology Weapon Plus stole from them. However, they are killed by Weapon XV.