Kanishka is known in history as a mighty conqueror, who not only ruled the territory belonging to his predecessor Kadphises II, but at the same time also made a vast addition to it. The spots where his coins and inscriptions were discovered, throw much light on the extent of the conquest of Kanishka. From the epigraphic records of his reign it is known that he ruled over the regions of U.P, Punjab, Northwestern Frontier Province and parts of Sind. The Peshawar Casket inscription "Sui-Bihar" and "Jeda inscription" testified his dominion over Northwestern India, Punjab, Northern Sindhu and Gandhara. The "Sarnath" and "Mathura" inscriptions confirmed his mastery over the regions of U.P. Kanishka`s inscription is also found in parts of Beneras. The inscription of Kanishka`s immediate successor was found in Sanchi of Malwa, which pointed out that the region of Malwa was under the influence of Kanishka.

The extent of Kanishka`s kingdom can also be determined from literary evidences. Al Beruni referred to Kanishka`s rule over Afghanistan and the adjacent parts of Central Asia. Hiuen Tsang mentioned the inclusion of Gandhara, Purushpura (Peshawar) and the country to the east of the Tsung-Ling Mountain in Kanishka`s kingdom. Dr. J.N. Bannerjee has pointed out that Kanishka`s conquest of Turkistan and taking hostage from the region happened in the early part of his reign. Chinese and Tibetan historians also have suggested that Kanishka conquered Eastern India. He annexed Ayodhya and Pataliputra and hence it appears from literary evidences that Kanishka pacified the country. Thus it is evident from literary records that the extent of Kanishka`s Empire was stretched upto the eastern limits, as far as Bihar. The Rajtarangini by Kalhana and other Buddhist traditions referred to the inclusion of Kashmir within his territory.

However there is a keen controversy among the scholars regarding Kanishka`s conquest of Bengal. According to the Buddhist annals, the areas covering the Rajmahal hills in Eastern India (i.e. Bengal) were included in the Kanishka`s Empire. The discovery of Kushana coins in Tamluk, Midnapore and also in Bogra, Murshidabad and Malda have strengthened the inclusion of the territory of Bengal within the realm of Kanishka. But Dr. D.C. Sirkar refutes the evidences supported by the coins. According to him these coins may not be genuine, or rather these coins were made in the local treasury for local use with the emblem of the then ruler, Kanishka.

Kanishka won undisputed suzerainty over a vast region. In due course he became the sovereign master of the entire Northern India. However his relation with the Saka-satraps is still a debatable question. Levi, Rapson and D.C Sirkar have suggested that Kshaharata Saka satrap Nahapana ruling over Western India and Deccan was a vassal of the Kushanas. The coins of Nahapana also bore Kushana influence. Moreover he used the Kanishka era, which later came to be known as the Saka era. But Dr.S.Chattopadhya has suggested that there is no evidence to prove that Kanishka extended his sway over Deccan. The inscription of Vashiska found in Sanchi throws much light on Kanishka`s relation with the other Malwa, the kingdom of the Sakas. Moreover other epigraphic and numismatics evidences available in the regions of Bhopal, confirms that Kanishka was the master over the entire region of Malwa.

Not only within India, Kanishka extended his boundary over the regions outside India. According to the Chinese annals, Kanishka defeated the king of Parthia in the west. The Chinese version of the Buddhist work `Sutralankara` states that Kanishka waged aggressive war and defeated the Parthian king. Though some scholars hold that the Parthian king, Pakoras resisted Kanishka`s attack, in general it is refuted by most of the historians. According to other scholars, Kanishka routed the Parthian king. Kanishka had already conquered his ancestral kingdom of Bactria and later he added parts of Parthia to it and in due course of time, he led a great expedition northwards. Kanishka conquered the Central Asian countries lying in the east of the Tsung-Ling Mountain. Smith also suggested that the kingdoms of Kashgarh, Khotan and Yarkhand were parts of the cast Empire of Kanishka. He also sent expeditions in the tributary states of China, to the west of the Yellow river. According to most historians, Kanishka probably earned this initial victory over the countries of Central Asia in the first part of his reign.

Towards the end of his reign, Kanishka`s authority over the areas of Central Asia was challenged by the sweeping victory of Pan-Chao, the general of the Chinese emperor, Ho-ti. Kanishka sent a massive army of 70,000 cavalry against Pan-Chao, which suffered terrible loss while passing through the hostile mountains. Ultimately the army was spreadeagled by Pan-Chao. According to Chinese tradition, Kanishka was compelled to pay tribute to the Chinese emperor Ho-ti and in the process lost his Central Asian kingdom. Though Smith and some other historians have opined that the Chinese Emperor Ho-ti defeated the Kushana king Kadphises II and not Kanishka, it is generally believed that the Kushana Empire suffered some diminution under Chinese pressure. However, the later scholars have opined that Kanishka suffered a defeat in the closing of his reign, after 23 years of his ascension. Romila Thapar, one of the famous interpreters of ancient history, opined that the Chinese army was so formidable and mighty, that Kanishka perished while fighting with such a powerful opposition. Thapar has based her conclusion on some annals preserved in the Chinese and Tibetan Writings. However this is a subject of great debate whether Kanishka died while fighting.

Kanishka was a great conqueror and his Empire reached its zenith, which was extended not only within the boundary of India but also areas outside Indian limits. He had established strong authority over the countries of Central Asia and the region of Parthia. In India, his Empire extended from Kashmir in the north to Sanchi in the south and from Beneras in the east to the Indus Valley in the west. According to some scholars the extent of Kanishka`s kingdom in the east extended upto the region of Bihar. Kanishka`s inscription and coins are found in the vast region of his Empire, which testifies the colossal conquests of Kanishka. Mathura was ascribed as the second capital of Kanishka.

Another group of scholars have discarded the above contention. They have pointed that the entire north India was not under the rule of Kanishka. According to them Panchala and Kausambhi were independent. The states of Kausambhi and Panchala were under the independent Mitra kings. They have again stated that the dates provided in the coins do not confirm that these places were included in Kanishka`s kingdom. Whatever the controversy is, it is now generally believed that Kanishka was a mighty conqueror, who was the master of entire northern India and also parts outside the periphery of India.