The best explanation of 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 highlights the key events in Asoka's life: his initial military conquest, the subsequent moral transformation he underwent, and his efforts to promote Buddhism and compassion as a ruler.