This is a repost, since I don't know how to do cool links like all you fancy-pants reddit power users 😉:

Buddhism was extremely widespread in the area (Afghanistan) prior to the arrival of Islam, and was probably the dominant religion, though virtually every other then-extant religion was represented in what is today Pashtunistan. It's not known for certain, however, what religion was dominant among the Pashtuns themselves, if they existed at that time at all. Ancient sources are relatively replete with references to 'Afghans' and 'Afghanistan,' but these are not proof of the existence of specifically Pashtuns. Afghanistan, today, is home to a large number of Iranian and Turkic ethnic groups, including Pashtuns, Hazara, Turkmen, and Tajiks, any of whom (except maybe the Hazara, who are a syncretic people) may have been the 'Afghans' of which the ancients spoke. Today, the majority of Pashtuns live in Pakistan, due to some arbitrary lines drawn by the British.

Nevertheless, after the arrival of Islam, its influence was fairly totalizing on the Pashtuns (or the people who would become the Pashtuns). The modern Pashtun way of life, Pakhtunwali, is very Islamic, and, yes, somewhat violent. To this day, it includes elements of what is known in the west as feud or vendetta.

But, the Buddhism practiced in the area did have lasting influence and did have some unique characteristics. Specifically, it had a notable influence upon the Greeks who settled there during the time of Alexander's conquests, and upon the states those Greeks founded after the fragmentation of the short-lived empire, such as Bactria and the Indo-Greek Kingdom. Indeed, it was essentially adopted as the official religion of the Hellenized elite in the area, and as a result there are, for example, sculptures of the Buddha done in the realistic Greek style, such as this one:https://upload.wikimedia.org/wikipedia/commons/e/e2/HellenisticCoupleWithBuddha.jpg. More than that, the Greeks, as they always did, heavily syncretized their religion with new ones, and translated many of their gods into Buddhist forms and translated many Buddhist figures into gods and heroes and the like.

The Indo-Greeks and Bactrians produced a huge amount of art and architecture (such as the many Yavana caves:https://upload.wikimedia.org/wikipedia/commons/2/20/Cave_17_at_Pandavleni.jpg; 'Yavana' is Sanskrit for 'Greek-speaker,' and inscriptions in these caves show that the Yavana were major sponsors of them), combining the culture of their original home with that of their new, much of which survives today in museums and on-site in central Asia. and which had an influence that outlasted the Greek kingdoms in the area, until its disappearance in approximately the 5th century AD, which was also just before the arrival of Islam, in the 7th century.

​

Further, and as to your actual question, Buddhism began to spread widely during the reign of Ashoka Maurya, who ruled most of India. He converted to Buddhism in 263 BC after a particularly horrific war, and had all the zeal you'd expect of a convert. He sent out emissaries and missionaries to the west to proselytize the religion. In this, he was assisted by, as you said, the southern route of the Silk Road through Persia (then ruled by the Seleucids, successors of Alexander, whose realm stretched from what is today Pakistan to the Aegean coast of Turkey), and by the Buddhist Indo-Greek kings and rulers on his northwest-border. Scholars generally agree that Mauryan emissaries were sent as far as the Mediterranean, but it is not certain whether these embassies included missionaries as well. Ashoka also sent emissaries east, and the religion spread to China, Mongolia, Korea, Japan and Indochina, as well, where it remains the majority religion (or ethical system, depending on your view of Buddhism) of many nations. In the west, as I said in my copypasta, the rise of Islam led eventually to the end of Buddhism west of India.