The best explanation for the events that led to the spread of Buddhism during Asoka's reign is:
Asoka took over Kalinga by force. Asoka rose to power through civil war. Asoka adopted Buddhism. Asoka became a just and compassionate ruler.
This sequence reflects that Asoka's violent conquest of Kalinga led him to be deeply affected by the suffering caused by war, which prompted him to adopt Buddhism and promote its values of peace and compassion, ultimately leading to its spread.