In 155 BC, a raid into Hispania Ulterior (Farther Spain) by the Lusitani and the defeat of two successive Roman praetors encouraged the town of Segeda in Hispania Citerior (Nearer Spain) to rebel. The following year, it refused to pay tribute or provide a military contingent to Rome but formed instead a confederacy with neighboring towns and began the construction of a defensive wall. Quintus Fulvius Nobilior was sent against the Celtiberians in 153 BC, with nearly 30,000 men. But the consul was late in arriving and ambushed soon after, with 6,000 Romans slain. A siege of Numantia several days later, where the Segedans had taken refuge, was no more successful. Three elephants were brought up against the town walls but became frightened and turned on the Romans, who retreated in confusion. There were other setbacks, and the hapless Nobilior was obliged to withdraw to camp, where more men suffered frostbite and died of the winter cold. Nobilior lost over 10,000 men in his campaign. In 137 BC, the Celtiberians forced the surrender of a 20,000-man Roman consular army led by Gaius Hostilius Mancinus. In 134 BC, the consul Scipio Aemilianus took charge of the demoralized Roman troops in Spain and laid siege to Numantia.