Excluding the Keterist battle-moons (which are more ISO than Capital Ship), these ships of the Mutual Progress Association are the largest warships ever built in Terragen history. The approach of a Juggernaut Class vessel is an awesome sight: a slender cylinder some 280 kilometres in length and twenty-five kilometres in diameter, accompanied by a swarm of giant autowars that are dwarfed to apparent mote-size by their parent vessel.
Like many other large-scale interstellar ships, Juggernaut Class vessels can change shape (attenuate) to pass through a wormhole mouth.
As might be expected, the MPA military hyperturings were quite enamoured with their giant dreadnoughts, which supplemented the earlier Leviathan and Gigantor Class. The first Juggernaut Class vessel was constructed to meet the Paradigm Threat and provide general fire support for PCO forces. Unfortunately, technical problems and continued lack of cooperation between the engineer and the designer hyperturings meant that by the time the Kiyoshi and eir sister ship, the Djed, (the Juggernauts, like the Gigantors and Leviathans before them, were all named after MPA Sectors; the Kiyoshi was the fourth vessel of that name, the Djed the fifth) was ready the war was almost over.
Although it played a minor role in the Battle for Drummon Duran (to pass through the wormhole the Dominion engineering corps had erected, the Kiyoshi had to attenuate to an astonishing 12000 kilometre length!) the Kiyoshi patrolled the Cygexba border for a while, more as an empire ship than to answer any real threat. Eventually the Juggernauts, which were far too expensive to maintain, even by MPA standards, were honourably discharged.
Following the great Cygexba sell-off, the Kiyoshi managed to procure a small nebula along the periphery, and set eirself up as a minor deity-worldship among the locals, while the Djed set up an independent polity in STC space.
The appearance of new threats - the Amalgamation and even more so the Laughter Hegemony - rekindled MPA interest in the Juggernaut Class vessel. To date, some fourteen have been built (the Kiyoshi came out of retirement to help oversee the construction of eir early sister vessels), and they presently patrol a wide swath of space in the Outer Volumes, mainly along corridors of Hegemony advance.
Until the turn of the eleventh millenium, the Juggernaut Class had never been bested in combat. During the Siege of Ar'Gor, a single Juggernaut Class ship, the Dravon, and twenty-five autowars held out against three entire Hegemony battlefleets. After twenty-five years and heavy losses, the Hegemony was forced to abandon any idea of ever intruding on MPA space, and shifted eir attention to harassing the outer STC volumes instead.
Even so, critics point out that the Juggernaut Class had never been tested against the Amalgamation other than to provide fire-support, or against a Keterist battle-moon, either of which would make a far more formidable foe. In view of the unlikelihood of a Keterist-MPA conflict (the two empires have had very close relations for the past several thousand years), and the fact that since the Amalgamation wars the MPA Command carefully kept their dreadnoughts away from Amalgamation space, neither of these ultimate tests is likely. Not until the start of the war with the Oracle Machines were these vast warships properly tested.
During the Oracle War two of the class were lost, the Medius and the Cableville, targeted by long range beam weapons. Only the fact that the Juggernaut class used powerful reactionless drives, and so could avoid beam weapon fire by constant evasive action, prevented more ships from being lost. However the vulnerability of these large ships to the beam weapons deployed by the Mystery Cult meant that they were less effective than expected.
Since the Oracle war many of these giant ships have retired or seceded from the MPA. Like the Leviathans and Gigantors, the Juggernauts are completely self-sustainable and self-repairing, and designed to function for centuries away from a home base. This is necessary considering the vast expanses of interstellar space they have to patrol, and the great distance from the nearest stargate (along with the inconvenience of having to attenuate to pass through a wormhole, always a delicate and time consuming operation). Often, especially after coming into contact with local and foreign ideas, memetic and noetic conditioning fails and the ship and eir complement secede. This has happened on at least half a dozen occasions, with the vessel in question setting up as an autonomous polity or embarking on a quixotic quest beyond the Periphery or otherwise following some course of action that is at odds with its original mission.