A series of invasions began in about 200 BC. The first to cross the Hindukush were the Greeks, who ruled Bactria in Afghanistan. That is why they are called the Indo-Greeks. The most famous Indo-Greek ruler was Menander (165-145 BC), also known as Milinda. He was converted to Buddhism by Nagasena. Menander asked him many questions regarding Buddhism before his conversion and his dialogue can be found in the Milinda Panho.
The Indo-Greeks were the first rulers in India to issue coins which can be definitely attributed to a king or a dynasty, unlike the punch-marked coins, which cannot be definitely credited to a king or dynasty. Further, they were the first to issue gold coins in India. The Greeks introduced many features of Hellenistic art in India. This fusion of two art forms is quite common and Gandhara Art is the best example of this fusion.
The Indo-Greeks were followed by the Shakas of whom the most famous ruler was Rudradaman I (AD 130-150). Although a foreigner, he was a great lover of Sanskrit and issued the first-ever long inscription in pure Sanskrit. The Shakas were followed the Parthians. Gondophernes was their most celebrated king in whose reign Thomas came to India.
After the Parthians, we see two successive Kushan dynasties. The first was founded by Kadphises who ruled for 28 years from AD 50. The Kadphises were followed by the Kushans. Kanishka was their most famous king, who had two capitals, Mathura and Peshawar (Purushupura). He, like Ashoka, was a strong champion of Buddhism and held a Buddhist Council in Kashmir, where the principles of Mahayana Buddhism (Hinayana Buddhism is the second sect) were finalized. Kanishka also started a new calendar known as the Saka Samvat in 78 AD. His empire was quite vast, extending up to parts of the modern-day Afghanistan and even Russia.
Kanishka was a great patron of art and literature and the great Sanskrit writer Ashvaghosh, who wrote the Buddhacharita, lived in the Kushan court. Vatsyayan's Kamasutra is a classic on sexuality. The science of Ayurveda took shape during these times. Charak, the founder of ayurvedic medicine and Susruta, the founder of ayurvedic surgery lived at this time and wrote two classic treatises Charaksamhita and Susrutasamhita respectively. The Indians learnt a lot from the Greek art and science. A large number of foreigners were absorbed into the Indian way of life.