Simplify the following text for a Grade 9 native English-speaking person.
Original: When the northern Netherlands revolted against King Philip II of Spain in the Dutch Revolt , the three northern quarters of Gelderland joined the Union of Utrecht and became part of the United Provinces upon the 1581 Act of Abjuration , while only the Upper Quarter remained a part of the Spanish Netherlands .
Simplified: The northern Netherlands stood up against King Philip II of Spain in the Dutch Revolt . The three northern quarters of Gelderland joined the Union of Utrecht and became part of the United Provinces when the 1581 Act of Abjuration was agreed .