The 1580 union of Spain and Portugal created history’s largest empire.

Evidence

Thus at a stroke the two great imperial powers were fused into one. Portugal’s domination of the African coast, the Indian Ocean, the East Indies and Brazil was now added to Spain’s hold upon parts of Europe and the Americas to form the largest empire the world had yet seen. — John Sugden, Sir Francis Drake, 613

Categorization

• corroborated: 2 independent passages support this claim — John Sugden, Sir Francis Drake, 613