paragraph: Alexander then chased Darius, first into Media, and then Parthia. The Persian king no longer controlled his own destiny, and was taken prisoner by Bessus, his Bactrian satrap and kinsman. As Alexander approached, Bessus had his men fatally stab the Great King and then declared himself Darius' successor as Artaxerxes V, before retreating into Central Asia to launch a guerrilla campaign against Alexander. Alexander buried Darius' remains next to his Achaemenid predecessors in a regal funeral. He claimed that, while dying, Darius had named him as his successor to the Achaemenid throne. The Achaemenid Empire is normally considered to have fallen with Darius. Alexander viewed Bessus as a usurper and set out to defeat him. This campaign, initially against Bessus, turned into a grand tour of central Asia. Alexander founded a series of new cities, all called Alexandria, including modern Kandahar in Afghanistan, and Alexandria Eschate ("The Furthest") in modern Tajikistan. The campaign took Alexander through Media, Parthia, Aria (West Afghanistan), Drangiana, Arachosia (South and Central Afghanistan), Bactria (North and Central Afghanistan), and Scythia. Spitamenes, who held an undefined position in the satrapy of Sogdiana, in 329 BC betrayed Bessus to Ptolemy, one of Alexander's trusted companions, and Bessus was executed. However, when, at some point later, Alexander was on the Jaxartes dealing with an incursion by a horse nomad army, Spitamenes raised Sogdiana in revolt. Alexander personally defeated the Scythians at the Battle of Jaxartes and immediately launched a campaign against Spitamenes, defeating him in the Battle of Gabai. After the defeat, Spitamenes was killed by his own men, who then sued for peace. 
question: Why did Alexander defeat Spitamenes at the Battle of Gabai?
answer: To get revenge for the death of Darius