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Who succeeded Guru Arjan as Sikh Guru?
After his execution by the Muslim officials of the Mughal Empire, his son Hargobind became the sixth Guru in 1606 CE.
So who won the dispute as the next Sikh Guru, succeeding Guru Arjan, and became the Sixth Guru ?
The mainstream Sikh tradition recognised Guru Arjan as the fifth Guru, and Hargobind as the sixth Guru
Are there any other interesting aspects about the Guru Arjan article?
Guru Ram Das chose Arjan to succeed him as the fifth Sikh Guru. His choice of successor led to disputes and internal divisions among the Sikhs.

Question: Why was there a dispute about succession ?
Self-contained rewrite: Why was there a dispute about succession of Guru Arjan as fifth Sikh Guru?