The battle of the Baetis (80 BC) was one of Sertorius’s first victories in Spain after his return from Africa and marked the start of the long Sertorian War.
Sertorius was one of the more able supporters of the Marian cause during Sulla’s Second Civil War, but when it was clear that the Marians were going to be defeated he left Italy and travelled to Spain, where he had been appointed governor. He was able to seize control of the province, but then had to flee to Africa after Sulla sent an army to depose him. After taking part in a civil war in Mauritania, he was invited back to Spain by the Lusitanians, who wanted him to lead a revolt against the Romans. Sertorius agreed to come back, but only if he was acknowledged as the legitimate Roman governor.
Sertorius landed at Baelo, west of Tarifa. He came with 2,600 of his original Roman supporters and 700 Libyans. After his arrival in Spain he was joined by 4,000 Lusitanian infantry and 700 cavalry.
Sertorius’s first major opponent was L. Fufidius, the Sullan governor of Further Spain. Plutarch records that Sertorius defeated Fufidius on the banks of the Baetis, killing 2,000 of his Roman soldiers.
In the same passage Plutach also records a naval victory over Cotta in the Straits of Gibraltar near Mellaria (on the African shore), a victory for his quaestor over Lucius Domitius, the governor of Nearer Spain, and a victory over Thoranius, who had been sent out by Metellus Pius. However Metellus was Consul in 80, and didn’t reach Spain until 79 BC, so these battles probably came at different times.
The Baetis was the Rome name for the Guadalquivir, the fifth longest river in Iberia, which rises in the Cazorla mountains, to the north-east of Sertorius’s landing point, and runs west then south-west, reaching the Atlantic in the Gulf of Cadiz, to the west of his landing point.
One surviving fragment of Sallust records that Fufidius arrived at a river bank with his legions, but found that the banks were steep and the ford would be difficult to cross if he had to fight. This probably relates to the fighting on the Baetis, and suggests that Sertorius arrived first, and Fufidius attempted an opposed river crossing and was defeated.