An interesting historical phenomenon is the fact that many of the major militaries based in India over the past millennium were eventually done in by non-Indian armies. There are, of course, numerous exceptions of note; for example, the Sikhs defeated the Afghans in the 1830s and the Marathas were victorious against the British in the first Anglo-Maratha War in the 18th century. Even when Indian armies won battles or successfully defended their territories, they often eventually lost wars in the long run because of their failure to follow up on victories, constantly fighting defensive wars.
Here are the three main problems with Indian armies over the past millennium:
Failure to Engage in Protracted Warfare
One characteristic of Indian armies was their inability or lack of desire to engage in protracted warfare, or guerrilla tactics. Ancient China in the warring states period featured the mass-mobilization of thousands of peasant soldiers who fought for years at a time, leading to enormous social disruption and famines. Yet, for better or worse, in India, local populations did not fight to the death and warfare remained the province of military elites. If these were defeated, kingdoms would often fall into enemy hands, as the entire Ganges Valley did after the defeat of a coalition of Indian princes by the Ghurid Empire in 1192. Within a year, these Ghurid armies reached Bengal at the other end of the valley and are said to have captured that region by walking in on the local king having lunch.
Yet even military elites rarely fought to the bitter end, preferring to make accommodations with their enemies. This was the case for most of the Rajput states during the Mughal period. A part of the problem stemmed from the lack of centralized command and unity of purpose in many Indian armies. If the main commanders were defeated or killed, the component parts of the army often quickly fell apart, even if victory was still possible. This happened, for example, at the Battle of Talikota in 1565, when the Vijayanagara Empire was defeated by a coalition of sultans. Although the city of Vijayanagar was not actually captured, and would have been difficult to do so, the various nobles of the empire simply fled back to their fiefs and abandoned the capital. Sieges were where local militaries had the greatest advantage, including the defense of a high ground, but as the Mughals learned in Rajasthan, terms were often most preferable to local rulers than mass starvation and defeat. However, when armies did engage in protracted tactics, as the Marathas did against the Mughal Emperor Aurangzeb for over two decades, there was success to be had.
No Change in Tactics
Before the defeat of Indian armies at Tarain in 1192, they had been battling Turkic militaries for several centuries, yet had failed to adapt their tactics accordingly. Turkic militaries used swift cavalry charges to defeat the largely infantry-based militaries of the subcontinent. Later on, in the 15th century, Indian armies, such as Vijayanagara and the Delhi Sultanate were slow to adopt the use of gunpowder despite guns being introduced in large quantities by the Mughals and Portuguese.
Additionally, for reasons beyond the comprehension of non-Indians, Indian armies continued to make heavy use of elephants. While elephants have some value in providing an initial shock, they are not very valuable in battle, as they lack the maneuverability and speed of horses and are as likely to trample friendly soldiers just as easily as enemy soldiers. Additionally as Nader Shah, leader of the Persians who defeated the Mughals in 1739 at Karnal pointed out:
What strange practice is this that the rulers of Hind have adopted? In the day of battle they ride on an elephant, and make themselves into a target for everybody!
By the time of their disastrous defeat at the Battle of Karnal against Persia in 1739, the Mughals had adopted many of the military characteristics of the subcontinent, including the use of large infantry armies, even while they maintained the swift cavalry that won them India in the first place in smaller numbers. This was partially a function of the heat of India which weakens horses. A recurring theme of Indian military history is the need for its states to import better horses from Central Asia and Arabia. However, despite all these, the Mughals and their successor states did not try to make the most of their infantries by equipping them with European-style training and weapons. At the Battle of Plassey in 1757, the British army, largely composed of 2,100 trained Hindu peasants and 1,100 British infantry defeated 50,000 Mughal cavalry.
No Power Projection
Indian armies largely remained overly defensive and failed to project power to take out their enemies. They instead aimed to weather attacks from outside of the subcontinent and hoped that their enemies would give up after failing to conquer India. The princes of the subcontinent faced constant invasion from the direction of Afghanistan for hundreds of years before the conquest of north India but never launched invasions attempting to subdue the Afghan cities of Ghazni and Ghor where these attacks originated from. Few Rajput states made any attempt to conquer and rule Delhi, despite being right next to it. India’s coastal states, some of which had navies, did little to clear European navies out of local waters, perhaps because they did not understand the strategic importance of the ocean.
The direction of conquest and the spread of authority was always projected into the subcontinent and rarely outside of it. Why was this? There are definitely logistical reasons behind this. It was very hard to move uphill and supply a large infantry based army moving out of the subcontinent. Mountain ranges make regular communication different. Additionally, here is little motivation for an army based in the fertile and warm subcontinent to conquer the barren mountains of Tibet or Afghanistan. The main reasons are strategic. Fortunately, some Indian-based empires, like the Mughals and Sikhs understood the importance of controlling the mountainous pathways into India and beyond.