The Alliance–Imperial War was the longest, bloodiest conflict in human history, lasting nearly 160 years, from 640 UC (331 IC / 3440 CE) to 799 UC (1 NIC / 490 IC / 3599 CE). The war ended with the Free Planets Alliance surrendering to the Galactic Empire and signing an unequal peace treaty. The Galactic Empire in turn shortly afterwards was replaced and succeeded by Reinhard von Lohengramm's New Galactic Empire.
The war between the Free Planets Alliance and the Galactic Empire began in 640 UC (331 IC / 3440 CE), when the Empire first learned of the existence of the Alliance. Emperor of the Galactic Empire, Kaiser Friedrich III, dispatched an invasion fleet through the Iserlohn Corridor, under the command of Grand Duke Herbert. The Free Planets Alliance countered with a fleet of its own, led by Lin Pao. Contrary to the Kaiser's expectation, Lin Pao's fleet won an overwhelming victory. The Alliance quickly established a defensive line within the Iserlohn Corridor, and the war with the Empire soon escalated, though neither side was initially able to gain a distinct advantage over the other. For much of the war, the Empire was on the offensive strategically, while the Alliance was fighting defensively to repel Imperial invasions.
At some point, a second navigable route between the Galactic Empire and the Free Planets Alliance was discovered: the Fezzan Corridor. Founded in 682 UC (373 IC / 3482 CE), Fezzan quickly established itself as an autonomous state, outside the jurisdiction of both the Empire and the Alliance — though technically it was still a dominion of the Empire. Fezzan became an independent zone, allowing for trade, diplomacy, and espionage to exist between the two nations.
- Battle of Dagon - First battle of the war. The Imperial Expeditionary Force is almost completely annihilated. Major Alliance victory.
- Battle of Shandarua - Alliance victory.
- Imperial Vice Admiral Telemann's troops mutiny.
- Imperial Admiral Martin Otto von Siegmeister defects to the Alliance.
- Battle of Forseti - Alliance victory.
- Battle of Firezard - Bruce Ashbey and the officers of the "730 Mafia" lead Alliance forces to victory. Major Alliance victory.
- Battle of Dragonia - Bruce Ashbey leads Alliance forces to another victory. Alliance victory.
- Second Battle of Tiamat - 2nd largest battle of the war. Death of Bruce Ashbey and effective disbandment of the "730 Mafia". Major Alliance victory.
- Battle of Palantia - Alliance's 11th Fleet takes severe damage. Imperial victory though diminished slightly by pursuit of withdrawing Imperial forces by Alliance reinforcements, resulting in more Imperial casualties.
- The Empire finishes construction of Iserlohn Fortress.
- Battle of El Facil - El Facil temporarily falls under Imperial control. Imperial victory.
- Battle of Arlesheim - The Keyserling Fleet opens fire prematurely, spoiling the Imperial ambush, and then suffers over 60% casualties from the Alliance counterattack. Alliance victory.
- Fifth Battle of Iserlohn - Alliance Fleet Admiral Sidney Sithole attempts to take Iserlohn Fortress with his parallel pursuit strategy but fails when the fortress fires through its own fleet to hit the Alliance forces. Imperial victory.
- Battle of Van-Fleet - Inconclusive space battle. Ground battle results in effective destruction of Alliance base on Van Fleet 4=2. Minor Imperial victory
- Sixth Battle of Iserlohn - Alliance missile cruisers bombard Iserlohn Fortress and inflict minor damage to the inner layer, but the Alliance offensive is ultimately repulsed. Imperial victory.
- Third Battle of Tiamat - Alliance's 11th Fleet sustains severe damage and loses its flagship. Imperial victory.
- Fourth Battle of Tiamat - Imperial victory.
- Battle of Astarte - Effective destruction of the Alliance's 2nd Fleet, 4th Fleet, and 6th Fleet. Major Imperial victory.
- Seventh Battle of Iserlohn - The Alliance's newly formed 13th Fleet captures Iserlohn Fortress with no losses. Major Alliance victory.
- Battle of Bilrost - The Alliance's 5th Fleet withdraws from the Reuenthal Fleet but sustains nearly 30% losses. Imperial victory.
- Battle of Dverger - The Alliance's 7th Fleet surrenders to the Kircheis Fleet. Imperial victory.
- Battle of Jafnhar - The Alliance's 13th Fleet gains the advantage over the Kempf Fleet and disengages. Alliance victory.
- Battle of Lesing - Effective destruction of the Alliance's 3rd Fleet by the Wahlen Fleet. Major Imperial victory.
- Battle of Lügen - The Alliance's 10th Fleet withdraws from combat with the Black Lancers but sustains 70% losses and loses Vice Admiral Ulanhu. Imperial victory.
- Battle of Wansteidt - The Alliance's 8th Fleet withdraws from the Mecklinger Fleet but sustains nearly 30% losses. Imperial victory.
- Battle of Alviss - The Alliance's 9th Fleet is ambushed by the Mittermeyer Fleet and withdraws, sustaining over 50% losses. Imperial victory.
- Battle of Bolthorn - The Alliance's 12th Fleet is effectively destroyed by the Lutz Fleet. Major Imperial victory.
- Battle of Amritsar - Effective destruction of the Alliance's 8th Fleet. Remnants of Alliance invasion fleet retreat to Iserlohn Fortress. Major Imperial victory.
- Eighth Battle of Iserlohn - The Yang Fleet and Iserlohn Fortress destroy Geiersburg Fortress, effectively destroy the Kempf Fleet, and inflict heavy damage to the Müller Fleet. Major Alliance victory.
- Ninth Battle of Iserlohn - The Yang Fleet evacuates and abandons Iserlohn Fortress which is then occupied by the Reuenthal Fleet. Imperial victory.