Bactrians were an Iranian tribe closely related to the Persians culturally and linguistically. I think Roxanne was Bactrian. Iranians were originally a nomadic people of the steppes. Some migrated south, to India (the Aryans), and some to Iran (Persians, Medes). Some migrated west (the Scythes). There were Iranian tribes in western China. Bactrians were a tribe that held the mountains in northern Iran and Afghanistan, and many of the lush farmland in the valleys. An ancient Northern Alliance if you will.
As I said, Alexander was a conservative. He started out to defeat the Persians to punish them for an invasion that occurred about 100 years early, and which still wounded Greek sensibilities, to free the Greek cities of Anatolia, and to deter future Persian aggression. Just like we protect ourselves from future aggression, by pacifying the lands of our enemies.
I looked up some of your past posts, and you have a keen interest in Balkan affairs, for whatever reason. I don't know if searches go back far enough, but you would see from my posts how harshly I criticized the Clinton Administration during the Serbian air strikes for doing a number of stupid things, including making enemies of former friends, supporting Islam in the heart of Europe, and using the US military to deflect memories from the sting of impeachment. I think that part of this current campaign will involve rooting out militant Islam from the Balkans, and reforming immigration policy in Germany, France, Britain as well as in the US. Nothing is stupider than creating a 5th column in our own back yards.