Afghanistan was subject to Persian rule. Badakhshan, the northern provinces, Juzjan (all the way to Takhar and Herat), Khurasan, Bukhara, and Samarqand were all part of the Persian empire. So, how did Uthman Bin Affan (may Allah be pleased with him) conquer it?
All these areas were conquered during the era of the third Caliph Uthman (may Allah be pleased with him). In the ‘Sunan’ of Abu Dawud, it says: “Abdul Rahman Bin Samurah narrated to us in Kabul...” It shows Abdul Rahman Bin Samurah must have conquered Kabul during the era of Umar or Uthman (may Allah be pleased with them).
None of the populations conquered by the Muslims ever rebelled or resisted, except for the people of Afghanistan. The people of Afghanistan are stubborn and it is not an easy task to make them submit. Muslims first conquered it during the era of Umar. It was followed by a rebellion from some tribes and the expulsion of Muslims. Muslims had to return later on and conquer Afghanistan a second time.
If Afghans are convinced of an idea or belief, they are known to cling on to it very tightly and spread it. They earlier followed the religion of Buddhism. There was no way to change them. Buddhism was it for them, which is why there are still statues of Buddha in Bamyan. Afghans spread Buddhism in the region.
Then Islam came. They were convinced with Islam and embraced it. They spread it throughout the region. So, most of the places around them adopted Islam through them. Mahmud Al-Ghaznawi invaded India seven times. He entered it and demolished the statue Shamnama. – Fi Dhilal Surat At-Tawbah, p. 141-142; translated by Tarek Mehanna