Rome's military and Senate approved Hadrian's succession, but four leading senators were unlawfully put to death soon after. They had opposed Hadrian or seemed to threaten his succession, and the Senate held him responsible for it and never forgave him. He earned further disapproval among the elite by abandoning Trajan's expansionist policies and territorial gains in Mesopotamia, Assyria, Armenia, and parts of Dacia. Hadrian preferred to invest in the development of stable, defensible borders and the unification of the empire's disparate peoples. He is known for building Hadrian's Wall, which marked the northern limit of Britannia.