Both being countries on the same peninsula, why didn't Spain annex Portugal?

On the Iberian Peninsula, Spain and Portugal, two countries with similar historical backgrounds, have markedly different developmental trajectories.

Compared to Spain, Portugal boasts a longer history.

Spain was formed by the later merging of Castile and Aragon, preceded by other kingdoms on the Iberian Peninsula like the Kingdom of León, which later became part of Castile.

Portugal coexisted with these kingdoms but avoided absorption into Spain, unlike others. This begs the question: why was Portugal an exception?

The Iberian Peninsula was once part of the Roman Empire during the early expansion of European civilization.

After the collapse of the Western Roman Empire, the Visigoths ruled the Iberian Peninsula, also adopting Christianity.

With the rise of the Arab Empire, Muslims targeted expansion towards the Iberian Peninsula. This led to Muslim rule in the south and Christian rule in the north, creating a unique scenario in the Middle Ages.

Despite northern Christian nations considering the south their "lost territories," conflicts between north and south persisted for centuries. Portugal took a leading role in the Reconquista, gaining independence from the Kingdom of León and initiating its own Reconquista efforts.

However, during this process, Portugal, like other nations, adopted a peculiar economic model known as "outward feudalism," where high military expenses relied on external expansion and plunder.

Portugal showed uniqueness in its Reconquista efforts due to its strategic geographic position, faster territorial gains compared to other nations, early nationalism sentiments, and differentiation from other countries.

This situation exacerbated tensions with Spain, particularly in territorial and trade competitions, leading to a series of conflicts.

After 1580, Spain became the most powerful country on the European continent, initiating the Spanish-Portuguese Union. However, this union was built on Spain's strength.

Portugal faced threats internally and externally, notably from the Dutch. Therefore, the Portuguese elite saw merging with Spain as the best choice to safeguard their interests, contrary to the nationalist sentiments of the Portuguese populace.

Over time, discontent with Spanish rule grew in Portugal, and Spain failed to meet Portuguese needs. Eventually, the Portuguese Restoration War erupted in 1640, with support from England, leading to the restoration of independence.

This event intensified tensions between the two countries, especially in international affairs. Portugal's independence marked the end of the dream of unifying the Iberian Peninsula.

In modern history, despite the twists and turns, Portugal and Spain have developed into entirely different countries. Their historical entanglements and political conflicts have rendered reunification impossible.