Final answer:
The principal source of revenue for the Roman imperial government and army from the first century C.E. onward was taxes, including those on agricultural land, individuals, imports, customs, and specific groups and communities.
Step-by-step explanation:
The principal source of revenue for the imperial government and the army from the first century C.E. onward was taxes. The Roman government collected taxes from various sources, including agricultural land, individuals, imports, customs, and specific groups and communities. These tax revenues were essential for conducting business, funding public programs, and sustaining the large armies needed for constant warfare.