The cloaking device was first developed by the Romulan Empire in the 2260s. Invisibility had long been possible - the selective bending of light around a vessel would render it invisible to whatever frequency range was selected. However, the power cost was so enormous that such a device was not considered to be practical. In 2266 the Enterprise encountered a Romulan vessel which carried the first known practical cloaking device. The ship was able to cloak itself from sight at will, and although it is was still visible to motion sensors these were not accurate enough for more than a general location - certainly not accurate enough for a firing solution. Although the cloak was quite effective, it did have its drawbacks. The power drain was so great that the Romulan vessel was unable to use its own scanners effectively and could not fire any weapons. The vessel also had relatively limited range due to the high fuel consumption. The Romulans apparently traded cloaking technology to the Klingons in exchange for warships. Certainly the Romulans began operating Klingon-designed vessels in the 2268, while the Klingons were known to employ cloaking technology at least as early as 2285. The Klingons made one major innovation in cloaking technology in 2293, when they produced a Bird of Prey capable of firing torpedoes whilst cloaked. The prototype vessel battled the USS Enterprise and USS Excelsior in orbit of Khittomer; despite emitting large amounts of neutron radiation the weapon was highly effective, but the Enterprise crew realized that the Bird of Prey could be tracked by following the plasma exhaust emitted by its engines and modified a photon torpedo to follow this trail. The development was so effective that the Klingon development was rendered next to useless, and was never again an effective combatant. In 2311 the Federation signed the Treaty of Algeron, which reaffirmed the Romulan Neutral Zone and forbade the Federation from developing or deploying any cloaking technology. Perhaps the most impressive example of cloaking technology was observed in 2364, when the supposedly mythical planet of Aldea was discovered by the Enterprise-D. The planet had been hidden for centuries behind a complicated light refracting mechanism which bent light around it in a similar manner to the Romulan cloaking device. The Minosians were also known to employ cloaking technology as part of their Echo Papa 607 weapons system. The Enterprise-D was forced to battle a cloaked drone weapon in orbit of the planet; it overcame the cloaking device by luring the drone into the atmosphere where the turbulence made it visible. Later in the same year the Romulan Warbird was encountered for the first time when the Enterprise-D encountered one at the Neutral Zone. It was initially believed that the Enterprise sensor systems could detect the Warbird through its cloak, but it quickly became apparent that the Romulans had in fact deliberately allowed themselves to be detected in order to determine the Federation's intentions towards them. Subsequent encounters revealed that the improved cloaking device still suffered from limitations. The Romulan vessels remain unable to fire whilst cloaked, whilst advanced in Federation sensor technology have made its invisibility less than assured. Subspace listening posts and gravitic sensors are both a worry for Romulan commanders, and the Federation has deployed large numbers of these along its borders. Perhaps the most effective development was the Tachyon detection grid; created by Enterprise-D personnel in 2368, the system involves a network of vessels which send tachyon beams towards one another across a large area of space. Even a cloaked vessel will disturb one of these beams when crossing it, hence rendering it detectable. In 2369 the Federation gained significant intelligence on the Warbird when a Starfleet officer masqueraded as a Romulan for a short time. She confirmed that the ship must keep a careful watch over its own electromagnetic missions whilst cloaked, and that the radiative emissions of the warp engines must be precisely balanced by means of nullifier cores. Even a small misalignment in one of these cores creates a polarized magnetic disturbance whenever the ship is in motion, a disturbance which can be detected by a Federation vessel. Ironically, perhaps the greatest single advance in cloaking technology was made by the Federation. In 2358 the Starship Pegasus was believed destroyed after a mutiny by the crew. However, in 2370 Starfleet discovered that the Pegasus was still largely intact. Admiral Pressman, Captain of the ship at the time of its loss, attempted to recover some technology from the vessel using the Enterprise-D. It transpired that certain Federation officers had been illegally developing a Phase Cloak, a device which could change the structure of matter in order render it both invisible and able to pass through normal matter. A vessel equipped with such a device could hide within a planet or a star, completely immune from conventional attack. The crew had been so fearful of what they considered to be a dangerous device that they had mutinied rather than continue to operate it. In 2368 the Enterprise assisted a Romulan vessel in distress, and discovered that this vessel had also been attempting to test a phase cloak. The results had been disastrous, causing an accident that would have completely destroyed the ship if not for the assistance of the Starfleet vessel. There has since been little sign of progress with this technology. In 2371 the Romulans agreed to allow Starfleet the use of a single cloaking device on board the USS Defiant in order to facilitate that ships missions against the Dominion, in return for all of Starfleet's information about the organization. Although the original agreement was to only use the cloak in the Gamma Quadrant, Captain Sisko subsequently violated this aspect of the agreement on many occasions.