Why did Caracalla move his army back to Hadrian's wall?

During Septimius

Severus's campaign to conquer Caledonia (Scotland), his son

Caracalla was the second in command of the army and his other son,

Geta, had administrative duties. Septimius Severus had taken the

Southern Uplands and the Central Lowlands and was preparing to

conquer the Highlands and wanted to exterminate the Caledonians.

However, he fell ill and died. The troops proclaimed both Caracalla

and Geta emperors. Caracalla abandoned the military campaign and

both brothers returned to Rome. This was because both brothers

wanted to be sole emperor and both wanted to concentrate on this

power struggle in Rome. Caracalla tried to murder Geta at a

festival of the Saturnalia (17 December) and later in that month he

got the imperial guard to kill him.