By Sanjeev Miglani NEW DELHI (Reuters) - - Indian security forces are preparing for one of their most challenging assignments in decades, protecting prime ministerial candidate Narendra Modi in a country with a grim history of political assassinations. A series of small bombs killed six people at a rally the Hindu nationalist leader held in the city of Patna on October 20. Authorities said the home-grown Indian Mujahideen (IM) group was responsible. While Modi was not in the immediate vicinity of the explosions, the message was clear.