Client kingdoms allowed Rome to secure volatile frontiers without maintaining garrisons.

Evidence

Where subjugation was likely to be challenging, client kings, dependent on Augustus’ goodwill but under an obligation to provide military aid, were the preferred option, since they removed the necessity for risky expansionist warfare or maintaining strong defences over a wide area. — Stephen Kershaw, A Brief History of the Roman Empire, 289

Categorization

• corroborated: 2 independent passages support this claim — Stephen Kershaw, A Brief History of the Roman Empire, 289