The Battle of Achenar, also known within the Empire as the Great Battle of Liberation, was an interstellar conflict between the Federation and the Empire that occurred in the early 24th century and was fought principally in the Achenar system. It was the first large-scale interstellar conflict in human history. Instigated by the Federation in an attempt to annex Achenar after the Empire repeatedly refused to join voluntarily, the Battle of Achenar ended in an upset victory for the Empire, which forced the Federal invasion fleet into retreat and harried it all the way back to its base in Beta Hydri. The war cemented the Empire's independence from the Federation and rise as a second superpower in the Core Systems, and resulted in a lingering animosity between both sides that would endure for almost a thousand years.
In the 23rd century, the wealthy, Earth-born, anti-Federation activist Marlin Duval assembled an expedition to establish a new colony beyond the Federation's reach. The expedition eventually arrived in Achenar, and in 2292, Marlin Duval founded the Republic of Achenar. Her idealistic vision of a representative government was short-lived, however, as she and her immediate family perished in an apparent speeder crash in 2296. Marlin's brother, Henson Duval, assumed control of the Republic's senate, then disbanded the ruling council and appointed himself the Republic's sole leader for life. Henson capitalised on the people's anti-Federation sentiment to advocate jingoism and authoritarianism, and his popularity grew. In 2320, Henson declared himself the first Emperor, and the Republic's transformation into the Empire was complete.
As Achenar thrived, it soon attracted the attention of the Federation, which repeatedly made diplomatic overtures to bring the colony into the fold under the Federal Accord, but was refused every time. Tensions came to a head when it was reported that colonisation and terraforming efforts on Achenar 6d had resulted in the displacement and extinction of the planet's native intelligent species, the mudlarks. The Federation came to believe that Henson Duval had deliberately ordered Achenar 6d's sterlization, but the Empire has long maintained that the mudlarks were the tragic victims of bacteria introduced to the world by colonists. Regardless of how the mudlarks died out, the Federation had strict ecological protection laws to prevent human colonists from destroying alien environments and species, and it asserted that the Empire had violated those laws and committed genocide, giving it a sufficient pretext to finally annex Achenar and replace its government with a friendly, compliant regime.
The Federation dispatched an invasion fleet under Admiral Richard Morgan from Beta Hydri to Achenar in late 2323, expecting the colony would be swiftly subjugated when the fleet arrived the following year. Unfortunately for the Federation, Achenar was alerted to the invasion and had ample time to prepare an ambush. When Morgan's fleet entered Achenar in 2324, it was dependent on a long and fragile supply line that stretched across a large swathe of unsettled space, leaving it with insufficient logistical support for its size. The Empire took advantage of this key weakness, and as Morgan attempted to confront the Empire in a single large battle, the Empire turned to pack-hunting formations to harass the Federal fleet before focusing on disrupting its supply line.
For the remainder of 2324, an opportunity for a decisive confrontation with the Empire's forces continued to elude Morgan, and all Federal attempts to establish a foothold in Achenar failed while his fleet dwindled from constant attrition. In 2325, Emperor Henson Duval pressed every available civilian and military ship into service to create a "People's Fleet" and ordered a massive counterattack that simultaneously assaulted every element of the Federal invasion force. Morgan was driven into retreat from the system, and his ships were constantly threatened by Imperial pursuers until they returned to Beta Hydri, where improved access to supplies halted the Imperial advance in its tracks.
Skirmishing between the Federation and the Empire continued sporadically in Beta Hydri until 2379, when Sirius Corporation began mediating negotiations to end what had evolved into a long and turbulent cold war. The Empire spent those decades expanding into Achenar's neighboring systems to create a buffer zone against further invasion attempts. Negotiations concluded in 2381, with the Federation agreeing to make significant concessions: Achenar and its colonies would be granted full autonomy, and official recognition of the Empire's hereditary title of Emperor.
The Battle of Achenar established the Empire as the second interstellar superpower within the human Core Systems and presented the Federation with its first real opposition. Emperor Henson Duval used his victory to solidify his rule, allowing him to mold the Empire's culture as he saw fit and creating traditions that would endure for centuries. At the conclusion of Federal-Imperial negotiations in 2381, Henson ordered the construction of the Hall of Martyrs beneath the Imperial Palace, a monument to the 16,032 citizens of Achenar who died in the "Great Battle of Liberation". In the Federation, Morgan's defeat had created a crisis in confidence in Earth-based leadership. This resulted in the Federation moving its capital from Washington, D.C. on Earth to Olympus Village on Mars, after a selection process that also considered locations outside of Sol, which mollified Federal colonies. The battle also gave rise to the infamously tense relationship between the Federation and the Empire, often described as an perpetual cold war in and of itself. Both superpowers would continue to compete with each other for territory and resources well into the 34th century.