Why did the early Caliphate initiate attacks on states like Armenia, Caucasian Albania?

Why did the Arab Caliphate under Umar – a very righteous and close companion of Muhammad ﷺ – attack first, for example, Armenia and Caucasian Albania? Is it permissible in Islam to attack non-believers who have not attacked first? After all, there is a hadith that states: "Whoever kills a Mu’ahid (a non-Muslim living in peace with Muslims), will not smell the fragrance of Paradise".