Banned by many Alpha and Beta Quadrant powers in the Second Khitomer Accords 
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Isolytic subspace weapons |
, these weapons are among the most dangerous ever conceived. The Accords compel all signatory governments to make all possible efforts to restrict the proliferation of subspace weapons, including restrictions on access to sensitive records; as a result information concerning them tends to be very hard to come by.
In the Federation, subspace weapons were first proposed in the 2330's. A low level research project was initiated, and when the researchers began achieving promising results the pace of development was stepped up in 2339. In the late 2330's and early 2340's relations with the Klingon Empire deteriorated significantly, to the point where it was widely predicted that war was imminent. 
The Federation Council pushed for a test weapon as soon as possible, and by 2343 a device was ready for detonation. The Federation's first subspace weapon was detonated by the Excelsior class USS T'Plek in the Sigma 387 system on Stardate 21347.8.
The weapon was an Isolitic device; many records have still not been made public, but from what has been published it appears that the subspace warhead was composed of a large matter-antimatter charge surrounded by a set of concentric shells composed of Verterium Cortenide and Tungsten-Cobalt-Magnsium. The same material as used in standard warp coils, these converted a large part of the detonation into a pulse of subspace radiation which would be of sufficient magnitude to cause a sympathetic detonation within the nacelles of any Starship within one light second. Such an secondary explosion would release more than enough energy to destroy the ship, while the subspace effect would be virtually unaffected by most known forms of shielding. Starfleet expected this weapon to boost the combat effectiveness of its Starships by at least an order of magnitude.
However, the Isolitic burst did not perform as predicted. Early in the project the development team had worried that such a large release of subspace energy would breach the space / subspace barrier, causing a tear which would allow the two sides to come into contact. Extensive research was conducted in order to determine the likelihood of this type of rift, and after considerable experimentation combined with theoretical models it was finally determined that the barrier between space and subspace was virtually impossible to breach using this form of weapon. Unfortunately, the test weapon detonated by the T'Plek formed a subspace tear immediately; the tear quickly began to spread across space towards the T'Plek.
The T'Plek attempted to evade and retreat at impulse speed, but the tear rapidly closed the distance; Captain Senak initiated warp drive to clear the area. The instant the warp drive was engaged the tear virtually exploded towards the T'Plek, enveloping and destroying the ship with all hands still aboard. Subsequent analysis indicated that the effect was attracted to the residual subspace radiation within the T'Pleks warp core and nacelles. With the strength of the attraction proportional to the intensity of the radiation, the T'Plek had done the worst possible thing by going to warp; the fully active warp core had dragged the tear across space at an estimated Warp 9.9992, almost equal to subspace radio speeds.
Despite the loss, Starfleet continued with its research throughout 2343. However, the weapon effects quickly proved themselves to be hugely unpredictable. 
Less than 5% of the devices tested performed as intended; some 60% simply exploded in a near normal matter/antimatter reaction, with no apparent subspace effect at all. The remaining 35% caused subspace tears which would then expand rapidly towards a nearby source of subspace energy. Even this effect was unpredictable; the research teams took to positioning large subspace radiation generators near the test areas in order to attract any subspace tear away from monitoring vessels. Despite these precautions, the USS Armin was almost lost when a tear headed for it rather than the decoys. Only the Captain's quick thinking in jettisoning both warp core and nacelles saved the ship from destruction. For subsequent tests purpose built monitors were provided which contained no subspace technology at all.After the loss of the Enterprise-C at Narendra III in 2344, tensions with the Klingons eased considerably. 
The second Khitomer conference was held in this year; at the arms control talks the Klingons announced that they, too, had been developing Isolitic subspace weapons - also with very limited success. It was agreed that these devices were simply too unpredictable for either side to employ, and production and deployment of these weapons was outlawed by the Second Khitomer Accords. 
Although initially applying only to the Federation and Klingons, virtually all other Alpha and Beta Quadrant powers have since signed up to this particular provision. Among the notable exceptions are the Son'a, an offshoot of the Ba'ku species who were reported to have fitted their warship fleet with subspace weapons. This was confirmed when a Son'a vessel fired an Isolitic burst at the Enterprise-E in 2375; a subspace tear was formed which headed directly towards the Enterprise. This ship was already suffering from battle damage at the time, and was unable to manoeuvre well as the battle was fought within a large set of anomalies known as the "Briar Patch" 
- not that being in open space would have made a great deal of difference...In the event, the crew ejected their warp core and detonated it inside the tear. 
This tactic had been suggested during the Federations research program, but the unpredictability of these weapons made the theory tenuous at best and no ship had the opportunity to test the idea out. Fortunately for the Enterprise, the tactic worked on this occasion and the ship was subsequently able to use the "Riker Manoeuvre" to defeat the Son'a attackers.