Celsus, P. Ma'rius
consul in A. D. 62 (Fasti
), was the commander of the fifteenth legion in Pannonia, with which he was sent to join Corbulo in his expedition against the Parthians in 64. On the death of Nero in 68, Celsus joined Galba's party, at which time he is spoken of as consul designatus, but whether he had been nominated to the consulship by Nero or by Galba is uncertain.
He was one of the ablest and most faithful of Galba's supporters; and when the troops rebelled against the new emperor, Celsus was sent to endeavour to propitiate the detachment of the Illyrian army which had encamped in the Vipsanian porticus.
It was probably thought that Celsus would have more influence with this army than any one else, on account of his former connexion with it: but he was unable to quell the insurrection.
The death of Galba soon followed, and Otho obtained the sovereignty.
The life of Celsus was now in great danger; the partizans of Otho loudly demanded his execution; but Otho, who appreciated his fidelity to his late master, not only spared his life, but admitted him to the circle of his most intimate friends. Celsus served Otho with the same fidelity as he had the late emperor.
He was sent, together with Suetonius Paullinus and Annius Gallus, in command of the army to oppose the generals of Vitellius, who were advancing into Italy.
At first he and his colleagues were completely successful; in the campaign on the Po, in the neighbourhood of Placentia and Cremona, they defeated all the plans of Caecina, the general of Vitellius [CAECINA, No. 9]; and it was not till the latter had been joined by Fabius Valens, and Otho had resolved, against the advice of Celsus as well as Suetonius Paullinus, to risk a battle, that the aspect of affairs was changed.
The battle of Bedriacum, in which Otho's army was defeated, gave Vitellius the empire; but Celsus, who had remained faithful to Otho to the last, again did not suffer for his fidelity. Vitellius allowed him to enter on the consulship on the calends of July (A. D. 69), as had been arranged fron the first. (Tac. Ann. 15.25
1.14, 31, 39, 45, 71, 77, 87, 90, 2.23, 33, 60.)