Excerpts from Books and Wikipedia
"Being subsumed into the Spanish Empire for sixty years--after Philip II invaded in 1581--hurt Portuguese pride but little else. A few Spanish garrisons were established, but it was never a military occupation. . . . What changed was Philip IV's attempts to introduce more Castilians to offices in Portugal to lessen their hardships as the Spanish-American economy declined and the Habsburg monarchy becoming embroiled in foreign wars. Portuguese interests were jettisoned and, to add insult to injury, they were forced to pay taxes to sustain these wars. . . . Dom Joao was proclaimed king, and the Spanish garrisons were driven out. Spain, with her troops tied up fighting the Dutch and French, could only look on." [Furtado: 1001 Days]