The Neutral Zone Incursion was a historical incident in 2266 when a Romulan Bird-of-Prey penetrated the Romulan Neutral Zone and destroyed a number of Earth Outpost Stations. The Romulans used the Neutral Zone as a testbed for a couple of new technologies, such as a cloaking device and a new type of energy weapon, and how these would serve them in a second Earth-Romulan War. It marked the end of a century of isolation on the part of the Romulan Star Empire. (TOS: "Balance of Terror")
On stardate 1709, the USS Enterprise was on patrol along the Romulan Neutral Zone, in Sector Z-6, when signals from Outpost 2 ceased. The starship set course for Outpost 4, the nearest of the outposts. About an hour later, signals from Outpost 3 also ceased.
When the Enterprise was a little over eight minutes away from Outpost 4 at maximum warp, the ship received word that the outpost was under attack by an unidentified spacecraft; contact was subsequently lost. As the Enterprise approached the border, Lieutenant Commander Spock was able to take sensor scans of Outposts 2 and 3; he determined that both outposts had been totally pulverized, including asteroids they had been constructed upon.
Five minutes out from Outpost 4, contact between the outpost and the Enterprise was reestablished. Commander Hansen, the commanding officer of the outpost, reported that Outpost 8 had also been destroyed. Outpost 4 had been hit by an unknown plasma weapon of immense power, that blew through the deflector shield with one shot. Even though the outpost itself was a mile deep within the asteroid, and protected by cast rodinium, the opening attack had caused immense physical damage to the outpost itself. The phaser banks were destroyed and the entire weapon crew killed with just the one hit. After firing, the mysterious starship had vanished, though it remained on tracking sensors.
As the Enterprise neared Outpost 4, the mysterious vessel reappeared for a moment, letting loose another plasma torpedo. Contact with the outpost was lost; Lieutenant Commander Spock reported it had been disintegrated. He theorized that the unknown ship was utilizing an invisibility screen to make scanning difficult, but that it had needed to become momentarily visible because such a high amount of power was required to maintain invisibility that it precluded firing weapons.
Pursuit back to the Neutral Zone
The vessel was only visible as a blip on tracking sensors; this blip, unaware of the Enterprise because of the two-way nature of the invisibility screen, changed course to 111 mark 14, which would take it back through the Neutral Zone to Romulus. The Enterprise followed the blip on a parallel course, hoping to make the other ship (now almost positively identified as Romulan) think it was nothing more than a sensor reflection.
When the Romulan vessel transmitted a coded message to Romulus, the Enterprise was momentarily able to tap into the visuals from the ship's bridge, obtaining the first-ever visual representation of a Romulan. The Romulans were apparently identical to Vulcans, which caused Enterprise navigator Lieutenant Stiles to view Spock with some suspicion. The invisibility system momentarily faltered as the Romulan ship turned, but power was reestablished, and the ship soon returned to its original course.
As the Enterprise and the Romulan vessel neared the Neutral Zone, Enterprise Captain James T. Kirk weighed his options. It was imperative that he stop the Romulan vessel from crossing back into Romulan space, for it would surely mean a second Earth-Romulan War if the Praetor felt the Romulan Star Empire had such a massive advantage.
At about twenty-one minutes from crossing into the Neutral Zone, the Romulan vessel suddenly changed course for comet Icarus IV. The plan of the Romulan commander was to obscure the Enterprise's sensors in the comet's tail so he could swing back and catch the Federation starship off guard. Captain Kirk failed to anticipate this move, however, and swung around to the opposite side of the comet, planning on attacking the Romulan vessel when it became visible as it entered the tail.
After the Romulan vessel exited the tail, its crew realized that the Enterprise was no longer behind it; the commander ordered escape maneuver one. Losing track of the Romulan vessel, the Enterprise turned hard to starboard. Kirk realized he had underestimated the Romulan commander, and ordered a firing patten laid down at random. The Romulan vessel suffered some minor damage, but the Enterprise's phaser control circuits overloaded and burnt out.
As the Romulan ship slowly became visible once more, Captain Kirk realized the danger, and ordered the Enterprise into emergency reverse warp, but he was too late; the Bird-of-Prey launched a plasma torpedo after the Enterprise. Even with the Enterprise's warp engines on full power, the torpedo was still capable of overtaking the starship.
Fortunately for the Enterprise, a little less than ten seconds before impact, the torpedo reached the limit of its range and dissipated, causing only minor damage to the ship.
The Romulan vessel reengaged its invisibility screen despite its fuel supply draining, and resumed its course on 111 mark 14; the Enterprise (its phasers once again operational) fell back onto its parallel course, going to maximum warp to catch up and hopefully intersect the intruder before it could cross the Neutral Zone border. With less than a minute to go, the Enterprise once again launched a number of random phaser barrages, hoping to force the Bird-of-Prey to decloak in order to fire.
Though the Romulan ship suffered some minor damage, its commander realized Kirk's strategy, however, and ordered all motion cut. Now no longer visible even on the Enterprise's tracking sensors, he had the ship's debris jettisoned into space, including the body of the only crewman to die thus far. The Enterprise detected the debris but realized there was not enough to account for the Romulan vessel. With nothing else to do, the Enterprise also cut all power and motion. Each ship's captain was trying to outwit the other, seeing who would "blink" first.
After almost ten hours of this waiting game, Lieutenant Commander Spock accidentally activated a device on his panel, while making repairs to the transfer coil of the Enterprise's phaser banks. Detecting the momentary signal, the Romulan ship moved underneath the Enterprise to attack it, but Captain Kirk anticipated the move and put his starship into reverse, laying down another random phaser barrage.
Rapidly running out of options as his ship's limited fuel supply ran low, the Romulan commander once again ordered debris jettisoned in space, this time ordering a nuclear warhead to be added to the mix. As the Enterprise approached the debris, it noticed the warhead too late: phasers managed to detonate it a meager 100 meters from the outer hull, sending around twenty crewmembers to sickbay with radiation burns.
The Enterprise was crippled in the attack, through overloads and circuit burnouts, and only the forward phaser control room was left operational, though it was manned only by Lieutenants Tomlinson and Stiles. Engine power was eventually restored, allowing the ship to move off and make repairs. Captain Kirk declined to do so, hoping that by maintaining the appearance of a tempting target, he could induce the Romulan ship to pull back to the Federation side of the Neutral Zone to attack. The Romulan commander, however, was reluctant to press his advantage, wary of draining his fuel supply, with the ship's fuel reserve having been entirely depleted. It was his hope the Enterprise would simply move off, allowing his ship to make for home.
The Romulan commander was, however, convinced by a subordinate named Decius to attack the Enterprise. As the Romulan vessel became visible once more and headed for the Enterprise, Kirk gave the order to fire. Unfortunately, a coolant leak had developed in the phaser control room, killing Tomlinson (the only Enterprise fatality in the entire battle) and knocking out Stiles. Luckily, Spock was in the area and, not affected by the gaseous phaser coolant as much because of his Vulcan physiology, he was able to fire the forward phasers.
The Romulan vessel was taken by surprise by the Enterprise's sudden barrage, and the ship was crippled beyond hope. Captain Kirk offered to take aboard the surviving crew, but rather than surrender, the Romulan commander performed his final duty and self-destructed his ship. In his last transmission, the commander told Kirk, "I regret that we meet in this way. You and I are of a kind. In a different reality, I could have called you friend."
With its flagship Bird-of-Prey an apparent failure, the Romulan Star Empire did not follow up on this incursion with a full-scale war against the Federation, though they did become involved in galactic affairs again, including Nimbus III, the Polaric Test Ban Treaty and even a brief alliance with the Klingon Empire in the 2260s. The observed tactic of sending a single ship to test an enemy's tactical and technological development and provoke a response was still considered a viable Romulan move almost a century later. (TNG: "The Neutral Zone")
The incursion revealed two very dangerous technologies at Romulan disposal. The first was their cloaking technology, which allowed them to penetrate the Neutral Zone. The device had a weakness, in that tracking sensors could pick up the ship when in motion, but this was neutralized in later generations of the device. The second technology was the plasma energy weapon. It was capable of outrunning a Constitution-class starship, but its effectiveness decreased with distance.
Though Captain Kirk had successfully avoided crossing into the Neutral Zone in his effort to stop the Romulan vessel, Starfleet Command's authorization to do so arrived hours after the battle had ended. (TOS: "Balance of Terror")
This conflict was invented by "Balance of Terror" writer Paul Schneider. "I had the concept of this battle in space and this battle over a neutral zone, and I sat there with Gene [Roddenberry] and developed it with him," he recalled. "Gene said, 'Take it this way, that way,' I added my bit and a story came out of it." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 34)
This confrontation between the Enterprise and a Romulan Bird-of-Prey was inspired by the World War II submarine movies The Enemy Below and Run Silent, Run Deep, with the Enterprise representing a "surface vessel" and the Bird-of-Prey, with its cloaking device, taking the part of a "submarine". (The Star Trek Compendium, 4th ed., p. 40) The reference book Star Trek: The Original Series 365 (p. 063) speculates that Run Silent, Run Deep may have inspired the idea of releasing wreckage and a dead body from a damaged vessel in an attempt to mislead the opposition, whereas The Enemy Below influenced the respect the two opposing captains gain for each other over the course of their encounter.