Both true and false. Most of the killing and destruction was done by the pagans to themselves. The pagan wars were no joke. Here in the Indian subcontinent itself there was a war called 'Kalinga War', the death and destruction was just so much and complete. Nothing remained standing.
https://en.wikipedia.org/wiki/Kalinga_War
This is just to be honest one of many hundreds to thousands of wars in India alone. Paganism was mostly destroyed by its own people.