The Oath of Abjuration or Plakkaat of Verlatinghe of July 26 1581, was the formal declaration of independence of the northern Low Countries from the Spanish king, Philip II.
This point meant a climax in the Dutch Revolt, a point of no return, in which the Low Countries were no longer loyal to their king.
Sometimes, the oath is interpreted nowadays as a secession from Spain, but this is not strictly speaking correct. Legally, the oath deposed the provinces' current ruler, Philip of Hapbsburg -- who, by dynastic coincidence, was also king of Spain. Philip's attempts to unify his various realms under a more centralized government based in Madrid were the major drivers behind the oath. Philip used troops from Spain and other areas loyal to him (such as Wallonia and Italy) to fight the rebels.
In the Oath, the Staten-Generaal (a sort of parliament) states that a king is a servant of his people and should respect their laws and traditions. When he no longer does this, the people have the right to choose another ruler.
In the declaration, these states are mentioned (in order of appearance):
Groningen, Overijssel and Drenthe also separated.
Philip II did not accept this Oath, and made the duke of Parma, Alexander Farnese, the landlord, who began a conquest with Walloon, Spanish an German troops that ended in the occupation of most of Flanders and the half of Brabant. The other countries gained their independence and became the Republic of the United Provinces (now: the Netherlands), while the occupied parts of Brabant and Flanders became the Spanish Netherlands together with the loyal Wallonia (now: Belgium)
The Oath stood model for the American declaration of independence in 1776 and those of the Southern Netherlands in 1789/1790 (the United Belgian States).