The best explanation for the events that led to the spread of Buddhism during Asoka’s reign is:
Asoka rose to power through civil war. Asoka took over Kalinga by force. Asoka adopted Buddhism. Asoka became a just and compassionate ruler.
This sequence highlights Asoka's violent rise to power and his subsequent transformation after witnessing the suffering caused by war, leading him to adopt Buddhism and promote its values of compassion and non-violence. This adoption greatly contributed to the spread of Buddhism during his reign.