Why do two groups claim to be the Taliban?

The Afghan Taliban came to prominence in its two-decade long war with the US and its ultimate victory back in August.

But there is an offshoot of the Taliban that operates across the border in Pakistan. Recently, Pakistan’s Prime Minister Imran Khan said that his government was in talks with members of the Tehreek-e-Taliban Pakistan or TTP.

Why are there two groups claiming to be the Taliban?