Current Historical Background

In 118 BC, King Micipsa of Numidia (current-day Algeria and Tunisia) died. He was succeeded by two legitimate sons, Adherbal and Hiempsal, and an illegitimate son, Jugurtha. Micipsa divided his kingdom between these three sons. Jugurtha, however, turned on his brothers, killing Hiempsal and driving Adherbal out of Numidia. Adherbal fled to Rome for assistance, and initially Rome mediated a division of the country between the two brothers. Eventually, Jugurtha renewed his offensive, leading to a long and inconclusive war with Rome. He also bribed several Roman commanders, and at least two tribunes, before and during the war. His nemesis, Gaius Marius, a legate from a virtually unknown provincial family, returned from the war in Numidia and was elected consul in 107 BC over the objections of the aristocratic senators. Marius invaded Numidia and brought the war to a quick end, capturing Jugurtha in the process. The apparent incompetence of the Senate, and the brilliance of Marius, had been put on full display. The populares party took full advantage of this opportunity by allying itself with Marius.
Populares ("favoring the people", singular popularis) were aristocratic leaders in the late Roman Republic who relied on the people's assemblies and tribunate to acquire political power. They are regarded in modern scholarship as in opposition to the optimates, who are identified with the conservative interests of a senatorial elite. The populares themselves, however, were also of senatorial rank and might be patricians or noble plebeians.
Populares addressed the problems of the urban plebs, particularly subsidizing a grain dole, and in general favored limiting slavery, since slavery took jobs from poor free citizens. They also garnered political support by attempts to expand citizenship to communities outside Rome and Italy.

The extensive campaigning abroad by Roman generals, and the rewarding of soldiers with plunder on these campaigns, led to a general trend of soldiers becoming increasingly loyal to their generals rather than to the state. Rome was also plagued by several slave uprisings during this period, in part because vast tracts of land had been given over to slave farming in which the slaves greatly outnumbered their Roman masters. In the last century BC at least twelve civil wars and rebellions occurred. The Second Servile War was a large unsuccessful slave uprising against the Roman Republic on the island of Sicily. The war lasted from 104 BC until 100 BC.
The Third Servile War: 73 BC – 71 BC in mainland Italy, led by Spartacus, did not happen yet (Current year is 90 BC).
Mithridates the Great was the ruler of Pontus, a large kingdom in Asia Minor (modern Turkey), from 120 to 63 BC. Mithridates antagonised Rome by seeking to expand his kingdom, and Rome for her part seemed equally keen for war and the spoils and prestige that it might bring.

Current military situation (Current year is 90 BC):

148 BC – Second battle of Pydna – The forces of the Macedonian pretender Andriscus are defeated by the Romans under Quintus Caecilius Metellus Macedonicus in the decisive engagement of the Fourth Macedonian War
146 BC – Battle of Carthage ends: Scipio Africanus Minor captures and destroys Carthage, ending the Third Punic War
Battle of Corinth – Romans under Lucius Mummius defeat the Achaean League forces of Critolaus, who is killed. Corinth is destroyed and Greece comes under direct Roman rule.
109 BC – Battle of the Rhone River – Roman force under Marcus Junius Silanus are defeated by the Helvetii (Gaulish tribe).
108 BC – Battle of the Muthul – Roman forces under Caecilius Metellus fight indecisively against the forces of Jugurtha of Numidia
107 BC – Battle of Burdigala – Roman forces under Lucius Cassius Longinus are defeated by the Helvetii.
105 BC, 6 October – Battle of Arausio – Cimbri (Germanic tribe) inflict a major defeat on the Roman army of Gnaeus Mallius Maximus
102 BC – Battle of Aquae Sextiae - Romans under Gaius Marius defeat Teutons (Germanic tribe).
101 BC – Battle of Vercellae – Romans under Gaius Marius defeat the Cimbri, who are entirely annihilated.
There is hostile tension between Rome and Pontus. Close to home, there is mounting tension between Roman Populares and Roman Optimates, which may escalate into civil war.