The Seven Dark Hours, as they are commonly called, was a period of seven hours that took place during the First Battle of Vespene, and was eventually the cause of the inconclusive battle.
On September 19, 2401, the first true UEG offensive into enemy territory began. The closest planet-designated object out of Sol, which, before the arrival of the Unity was going to be explored by a recon fleet, called Vespene, was a Nitomian planet with a strong military prescence on it. So, a joint fleet under the command of Fleet Admiral Hector von Brown (who was present at the First Battle of Mars, the First Battle of Earth and the Second Battle of Ganymede) and Fleet Admiral John Harper, commanding the 2nd Fleet and portions of the 3rd Fleet, moved to attack the red-desert planet. Upon entry in the planet's gravity well, contact was made with a small defensive fleet of about 20 ships, which was almost immediately neutralized with little losses. With part of the mission already completed, the fleet entered the atmosphere of the planet*. The planet was one massive desert, dotted with mesas, cannons and plateaus. The local area in which the invasion took place was formed mainly out of a dozen or so mesas in a loose circle, in the center lying the true Nitomian military base with heavily dug-in troops. The ships were to land their ground detachments to invade their own mesa (frigates were usually in pairs due to their small size). Inside the mesas, however, was the most important part. The Nitomians had carved out entire outposts inside each mesa, with several hangars including fighters, bombers, and transports, all guarded by roughly a battalion ground troops. As soon as all the mesas were taken (with another large portion of Marines on the "Desert Floor" fighting the main garrison), Two Fleet was ordered in to land their reinforcements while the troops in the mesas were to go down to help the assault on the Nitomian base. However, just as Two Fleet was to launch their marine contingents, a Nitomian strategical bomber flew by overhead, dropping a nuclear warhead in the center of the Desert Floor. Half of One Fleet was vaporized along with the Nitomian base and the invading marines. As the explosion began to suck in any nearby objects, another large porion of One Fleet crashed into both mesas and the ground.
Shortly afterward, USN Harbinger made contact with her Marine battalion, the 1st Marine Battalion. They were told to move to the top of their mesa (designated mesa 1B) where the rest of the battalion was gathering wounded and trying to link up with other troops. Harbinger, however then departed to link up with the almost untouched Two Fleet commanded by Fleet Admiral John Harper and the unknown fate of Fleet Admiral von Brown.
During the seven following hours, the surviving naval forces struggled to reunite with each other, all at the same time receiving hundreds to thousands of wounded marines from the planet's surface. However, when a portion of 1st Battalion was ordered to scout the wreckage-strewn Desert Floor for any survivors (which was in fact impossible), they made contact with yet more Nitomian soldiers, a recon picket. They were immediately wiped out. Admiral Harper, knowing that there were no other Nitomian bases on the planet at the time, correctly guessed there had to be a stealth ship in orbit. Ordering the bulk of Two Fleet back out into the gravity well to investigate, they picked up three Unity stealth ships that were ferrying troops down planetside. The ships were ordered destroyed immediately.
As the remains of the Marine battalions and divisions on the planet were brought aboard the remains of the attack fleet, they pulled back and left for Earth. Upon arrival, Admiral Harper and (previously thought dead) Admiral von Brown told Chancellor Samuel Acre of the disastrous defeat.
- "Chancellor, sir, from out one-hundred ships just forty remain. They deployed a nuclear device right at the center of our ground troops, it annihilated our troops and our ships. It took us hours just to reorganize. Seven dark hours that will haunt me forever."
- —Fleet Admiral John Harper to Chancellor Samuel Acre, reporting of the First Battle of Vespene
Admiral Harper's words would eventually give the terrible event a name, immortalizing it. Acre would later note, after the end of the war, "It was humanity's darkest hours."
- The fleet was separated into two flotillas: One Fleet and Two Fleet (almost all of One Fleet was decimated during the beginning of the Seven Dark Hours, and Two Fleet helped in leading the recovery efforts).