After the arrival of Europeans, the greatest number of Indians died as a result of

As a result of diseases.

Explanation:

The Spanish and other Europeans that arrived in the New World in the 16th century brought diseases that were unknown among the Indian peoples and for which the local human population did not have anti-bodies, immunological resistance. Good examples are smallpox and siphyllis. It is impossible to know today how many Indians died because of sickness caused by the new viruses and bacteria, but some estimates put it in some 40-60 millions deaths, or some sources put even around 100 million deaths.

The war that happened between them