[6](p 68) When Julius Caesar became governor of Cisalpine Gaul in 58 BC, he inherited four legions, numbered VII, VIII, IX, X, that were already based there.

With Octavian, whom the Senate later titled Augustus, established as sole ruler of the Roman world, the legion was sent to Hispania to take part in the large-scale campaign against the Cantabrians (25–19 BC).

Following the abandonment of the eastern Rhine area (after the disastrous Battle of the Teutoburg Forest in AD 9), the Ninth was relocated in Pannonia.

In AD 43, the legion most likely participated in the Roman invasion of Britain led by the emperor Claudius and general Aulus Plautius, because they soon appear amongst the provincial garrison.

In desperate hand-to-hand fighting the Caledonians entered the camp, but Agricola was able to send cavalry to relieve the legion.

Seeing the relief force, "the men of the Ninth Legion recovered their spirit, and sure of their safety, fought for glory", pushing back the Caledonians.

[3] Several inscriptions attesting IX Hispana have been found in the site of the legionary fortress on the lower Rhine river at Noviomagus Batavorum (Nijmegen, Netherlands).

These include some tile-stamps (dated 104–120); and a silver-plated bronze pendant, found in the 1990s, that was part of a phalera (military medal), with "LEG HISP IX" inscribed on the reverse.

[10] In addition, an altar to Apollo, dating from this period, was found at nearby Aquae Granni (Aachen, Germany), erected in fulfillment of a vow, by Lucius Latinius Macer, who describes himself as primus pilus (chief centurion) and as praefectus castrorum ("prefect of the camp", i.e. third-in-command) of IX Hispana.

[12] However, it is unclear whether this detachment was drawn from the IX Hispana (and its attached auxiliary regiments) alone, or from a mix of various British-based units.

According to the eminent 19th-century German classicist Theodor Mommsen, "under Hadrian there was a terrible catastrophe here, apparently an attack on the fortress of Eboracum [York] and the annihilation of the legion stationed there, the very same Ninth that had fought so unluckily in the Boudican revolt.

Mommsen cited as evidence the Roman historian Marcus Cornelius Fronto, writing in the AD 160s, who told the emperor Marcus Aurelius: "Indeed, when your grandfather Hadrian held imperial power, what great numbers of soldiers were killed by the Jews, what great numbers by the Britons".

As a result of these, and of inscriptions proving that two senior officers, who were deputy commanders of the Ninth in c. 120, lived on for several decades to lead distinguished public careers, led to the Mommsen theory falling out of favour with many scholars.

These now suggest later conflicts in other theatres as possible scenes of IX Hispana's demise: Several scholars continue to argue that destruction in Britain is the most likely scenario for the Ninth's disappearance.

Russell argues that "by far the most plausible answer to the question 'what happened to the Ninth' is that they fought and died in Britain, disappearing in the late 110s or early 120s when the province was in disarray".

Keppie suggests that it was the legion's absence elsewhere that encouraged a native uprising, obliging Hadrian to send the Legio VI Victrix to Britain.