Although the Company ruled the areas of their factories and trading posts, the beginning of true British authority in India is often dated from the 1757 Battle of Plassey. In this battle the British faced off against the Nawab of Bengal and French allies. It solidified the Company's power in the region, and as a result annexed all of Bengal.
This victory was compounded at the Battle of Buxar, in which Shah Alam II, Emperor of the Mughal Empire, was decisively defeated by the much smaller army of the Company. The Company won over 100 pieces of artillery and over one million rupees. This later resulted in the Treaty of Allahabad, which granted the company revenue rights to over 400,000 square kilometers of land. In addition to this, the defeated natives were forced to pay over 5 million rupees in war indemnities. While the Battle of Plessey cemented British imperial power in India, the Battle of Buxar is what truly established the Company as the dominant force in India.
In 1766 the first of the four Anglo-Mysore Wars began. The British were in alliance with Mysore, and they supplied Mysore with weapons and troops. The British and Mysore attacked a confederacy of other Indian states. This first war came to a quick close after three years of brutal fighting, ending in a draw in 1769 with no clear victory. No territories exchanged hands and everything continued as before the war.
In 1780 Second Anglo-Mysore War began after Haider Ali, the ruler of Mysore, invaded British controlled Karnataka. He blamed the British for the stalemate of the First Anglo-Mysore War. After four years of warfare, the Treaty of Mangalore was drafted. After having no clear winner, the two countries and their allies agreed to a status quo ante bellum (the state in which things were before the war) in 1784.
The Third Anglo-Mysore War began when the Mysore ruler, Tipu Sultan, invaded the close by state of Travancore, which was a British ally in the region. A war resulted between the two and a vicious three years of conflict quickly followed the invasion of Travancore. The war was concluded after the British siege of Seringapatnam by the British. The Treaty of Seringapatnam soon followed in 1792 with The Company as the clear winner. Mysore ceded over half of its territory to the Company along with the Emperor's two sons as war hostages.
The fourth and final Anglo-Mysore War was a reaction to the rise of Napoleon in 1799. Mysore, being a strong ally of the French, was key to Napoleon's plan to take over British colonies in India. The British invaded and Mysore and laid siege to its capitol, Srirangapatnam after a few engagements with Mysore's army. During the siege the Mysore ruler, Tipu, was killed. The Company, as a result of the capture of Srirangapatnam and the death of Tipu, the Kingdom of Mysore became a princely state in British India with its sovereignty as dead as their ruler.
The Company's dominance in India can be easily explained. By allying themselves with weak local governments they were able to gain small footholds in India. Once they had secured a place upon the subcontinent, they could begin to make demands of each small state. After 250 years of slowly annexing tiny native kingdoms, the Company was able to unite the land under the British flag.
