The Union of Utrecht was signed in May 1579
by the states of Holland
. It was a defensive alliance formed in response to the Union of Arras
signed earlier that year by Flanders
that pledged to uphold Catholicism
. It became the base for the rebel fight against the Spanish crown in The Dutch Revolt
and in the end became the independent Dutch Republic
The states entered into discussions at the end of 1578/beginning of 1579 and an outline agreement was reached. However William of Orange was unhappy with the proposal, mostly because it would spell the end of the Pacification of Ghent, his greatest political achievement. He held out and refused to sign the treaty. However in January 1579 the Union of Arras was signed forming an alliance of Southern Catholic states. This prompted Orange to realise that the Pacification was doomed anyway so he signed the treaty.
The United Provinces, as they became known, were to act "as if they were a single province" in matters of war and peace. No other policies were controlled centrally and each state was still free to chose its own religion. However despite this apparent commitment to freedom of religion the well organised and influential Calvinists, still in the minority (only about 10%), persecuted Catholics successfully and forced the establishment of entirely Calvinist provinces.
The Union drew up the boundaries between the Northern provinces and the Southern provinces which had reunited themselves with the King of Spain, Philip II, thanks mostly to his Governor General, the Duke of Parma. These borders obviously fluctuated during the ensuing fighting between the rebels and the Spanish provinces however by the Twelve Year Truce of 1609 the rebels held all of the land which came under the Union's jurisdiction and some more on top of that.
My own notes made in class
"The Netherlands: Revolt and Independence, 1550-1650" - Martyn Rady, Arnold 1987
"The Dutch Revolt, 1559-1648" - Peter Limm, Longman 1989