The Act of Federation of 1576, (popularly known as the Union of Delft) was signed on 25 April 1576 by William the Silent (the then Prince of Orange) and the provinces of Holland and Zeeland in the Netherlands and made a definitive federation of the two provinces. It also gave William supreme authority in war, allowed him to call a States General when he pleased, and effectively made him head of it. This allowed him to better gather resources and plan for attacks on the invading Spanish.
Some of stadtholders of the provinces saw the Union of Delft as a threat to their power. The three Stadtholders, Lambers, Van Gessel and De Witte tried to turn their provinces against the Union, but they were halted in their tracks by the mastermind behind the Union, Martin Drent.