Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Diaeus, who after the fall of Critolaus had resumed the chief command, summoned all men capable of bearing arms to the isthmus, and ordered 12,000 slaves, natives of Greece, to be enrolled in the army; the rich were applied to for advances, and the ranks of the friends of peace, so far as they did not purchase their lives by bribing the ruling agents in this reign of terror, were thinned by bloody prosecutions. The war accordingly was continued, and after the same style. The Achaean vanguard, which, 4000 strong, was stationed under Alcamenes at Megara, dispersed as soon as it saw the Roman standards. Metellus was just about to order an attack upon the main force on the isthmus, when the consul Lucius Mummius with a few attendants arrived at the Roman head-quarters and took the command.
Meanwhile the Achaeans, emboldened by a successful attack on the too incautious Roman outposts, offered battle to the Roman army, which was about twice as strong, at Leucopetra on the isthmus. The Romans were not slow to accept it. At the very first the Achaean horsemen broke off en masse before the Roman cavalry of six times their strength; the hoplites withstood the enemy till a flank attack by the Roman select corps brought confusion also into their ranks. This terminated the resistance. Diaeus fled to his home, put his wife to death, and took poison himself. All the cities submitted without opposition; and even the impregnable Corinth, into which Mummius for three days hesitated to enter because he feared an ambush, was occupied by the Romans without a blow.
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Reference address : https://www.ellopos.net/elpenor/rome/4-01-gracchi.asp?pg=81