Parthian activity had forced Demetrius II to take action in 139 BCE.

Marching against Mithridates I, king of Parthia, he was initially successful, but is defeated in the Iranian mountains and taken prisoner the following year.

This effectively ends Seleucid claims to any land east of the Euphrates river.

The Babylonian province of the Seleucid empire becomes Parthian, but …

…in Syria the dynasty's grip is reassured under Antiochus VII Sidetes, the younger brother of Demetrius, who also marries Cleopatra Thea.

Antiochus, having put Trypho to flight and ended his career, ...

…delivers an ultimatum in 138 BCE to the Jews to acknowledge him as overlord.

When they refuse, he sends an army against them, led by his general, Cendebeus.

Simon's sons Judas and John command the force that repels the Seleucid invasion of Judah in 137.

The Numantians themselves fight on, forcing the surrender, in 137 BCE, of a twenty thousand-man Roman consular army led by Gaius Hostilius Mancinus, Roman consul for this year, who allegedly put out his fires and tried to flee by night before being surrounded and forced to make peace.

According to Plutarch, Tiberius Gracchus was instrumental in bringing about the peace and saving twenty thousand Roman soldiers.

He returns home something of a hero, but Mancinus is put on trial by the Senate, which refuses to accept the treaty.

While Gracchus and other lieutenants are saved by Scipio Africanus Minor, the Senate decrees that Mancinus be handed over to the Numantines, as some twenty Roman commanders were handed over to the Samnites after the defeat at Caudine Forks in 321 BCE.

Plutarch does not relate Mancinus' further fate; Appian, however, noted that he was taken to Spain and handed over naked to the Numantines, but that they refused to accept him.

The Roman Senate, however, refuses to acknowledge defeat and elects to continue the struggle.

Tiberius Sempronius Gracchus takes offense when his brother-in-law, Scipio Aemilanus Africanus Minor, secures the rejection of the treaty that Sempronius had negotiated to save a Roman army from surrender to the Celtiberians at Numantia.

After the final Punic War, the Romans shift their focus to the conquest of Hispania.

Following the death of Viriathus, the Lusitanian chieftain, and the subsequent defeat of his successor, Tantalus, by Quintus Servilius Caepio (consul in 140 BCE), the Roman proconsul Decimus Junius Brutus leads the campaign to conquer western Iberia.

Brutus secures major victories in the south of modern Portugal before advancing northward. Along his march, he fortifies Olissipo (modern Lisbon) and systematically destroys settlements, expanding Roman control over the region.

In 137 BCE, the Roman campaign in western Iberia advances as Decimus Junius Brutus captures Talabriga (modern Marnel, near Águeda).

Brutus then fortifies Vissaium (modern Viseu), crosses the Douro River, and pushes further north to reach the Lima River.

According to Strabo, Brutus ultimately reaches the Minho River, extending Roman control deep into Gallaeci territory.

That same year, the Gallaeci, reportedly sixty thousand strong (as recorded by Paulus Orosius), confront the Roman legions at the Douro River. The ensuing battle results in a decisive Roman victory, after which Brutus returns to Rome as a celebrated hero. In recognition of his conquest, he is granted the agnomen Gallaicus, meaning "conqueror of the Gallaeci."