One of the largest elephant threats over time has been due to the ivory trade. Ivory is unique to elephants and their tusks are the only source. Unfortunately elephants also have a long life span and do not breed or grow quickly. Therefore overhunting for ivory largely depleted the elephant population. While hunting has been curbed it is still possible to hunt the elephant in many countries as long as you get a permit.
The vast majority of elephants were killed for their tusks as they were the only source of ivory. Most countries have banned elephant hunting but a few still allow hunting by permit only.
Elephants need a huge tone of food each day as well as large tracts of land to travel. By decreasing their natural habitats this in turn decreases the number of elephants that can survive in the wild as food and other resources become scarce.
Deforestation completely kills off their habitats in such a way that it cannot recover. Deforestation also makes water scarce and reduces the shrubs and trees. What little is left is quickly consumed by the elephants until the ground cannot support any animal life. This search for food is extensive and elephants need huge tracks of land.
Even though national parks are meant to try and save elephants they can actually be a threat. This puts boundaries on their land and can cut them off from some of their seasonal feeding grounds as well as the areas that they may normally breed. This can cause the elephants to stop breeding or even die because they cannot access the lands they need to.
Overcrowding has become a problem in parks so this may not be a feasible option for trying to save elephants. Many national parks actually have to cull elephants in order to control the population. Elephants are also threatened by domestication for work in which many are ill treated.