Cause
The increasing power of Guru Gobind Singh, and his establishment of a military order (Khalsa) alarmed the Rajas of the Sivalik hills. After some failed attempts to check the Guru's power, the Rajas pleaded the Mughals to help them against the Guru.
The Mughal viceroy of Delhi sent his generals Din Beg and Painda Khan, each with an army of five thousand men, to subdue the Guru. The Mughal forces were joined by the armies of the hill chiefs at Rupar. The Guru appointed the Panj Piare, his five beloved Sikhs, as the generals of his army.
Read more about this topic: First Battle Of Anandpur