How did rome keep such a large empiire unfied?

Answer 1

Rome kept such a large empire unified through tolerance, proving benefits, Propaganda and the army.

The Romans tolerated the religions and customs of the conquered peoples. They allowed them to continue to worship their religions, follow their customs and use their customary laws at the local level. They also let the local ruling classes run most of the local affairs. The role of the provincial governors of the provinces (conquered territories) was restricted to defence and maintenance of the legions stationed in the provinces, tax collection, public works and the arbitration of disputes the locals could not resolve by themselves. This policy had two advantages: it reduced the administrative load of the provincial governors and it facilitated the integration of the locals into the ideology and the economy of the empire. A less tolerant policy would have led to too many rebellions and would have threatened the stability of the empire.

Being part of the empire also provided economic and, sometimes, security benefits. The empire developed thriving trading networks and the Romans encouraged the peoples in the provinces to increase the production of manufactures agricultural goods for trade. The wealthy and middling class benefitted from this. The exploited poor did not. The Roman legions stationed in the provinces helped with maintaining order. In the frontier areas they also provided protection from raids from across the border. The troops were also customers for local traders.

The Romans fostered an imperial ideology to help to promote as sense of affinity and uniformity among the peoples in the empire.

The Romans army was also used to suppress rebellions. The Romans were ruthless in dealing with rebellion.