Philip II Acclaimed King of Portugal at the Cortes of Tomar, 1581
The Cortes of Tomar accepting Philip II of Spain as Philip I of Portugal in April 1581 — the settlement that created the Iberian Union. Engraved more than a century after the event by the Amsterdam printmaker Jan Luyken, so the ceremonial detail is his reconstruction rather than a record of the day.
The union of the two crowns was not a conquest presented as one. At Tomar in April 1581 the Portuguese Cortes accepted Philip II of Spain as their king on written terms: Portuguese offices for Portuguese subjects, a separate coinage, separate courts, and a monopoly on the empire's trade that Castile was not to touch. Those terms are the Estatutos de Tomar, and they are why the arrangement is called a union rather than an annexation. Whether they were kept is the question the article turns on. See what the Iberian Union was, how the shared crown worked, and why it collapsed in 1640.
Public domain — source (Wikimedia Commons). Free to reuse under the stated licence.