The reasons for the power of Rome.

after a long decline, wise rulers came to power who tried to bring Rome into a dignified form, throwing a large amount of resources into urban planning, and abandoned wars while the empire’s politics and economy were rehabilitated. also there was a reconciliation with the Senate, which allowed to resolve the internal political conflict.
in addition, Rome opened its borders to any people, which made it possible to attach other conquered empires to the culture of Rome, therefore, it expanded its borders due to spiritual influence and ultimately became the main modern civilization at that time.