Allowing the Goths in was a tactical blunder for Valens.

Evidence

The Romans had missed a golden opportunity: as allies, the Goths could have been the source of strength that Valens had hoped for; as enemies, they would ultimately bring Rome down. — Stephen Kershaw, A Brief History of the Roman Empire, 1707

Categorization

• corroborated: 2 independent passages support this claim — Stephen Kershaw, A Brief History of the Roman Empire, 1707