The Union of Arms (in Spanish Unión de Armas) was a political proposal, put forward by Gaspar de Guzmán, Count-Duke of Olivares for greater military co-operation between the constituent parts of the composite monarchy ruled by Philip IV of Spain.
The plan was for each of the kingdoms ruled by Philip to contribute equitably to a fund from which 140,000 troops would be maintained for the defense of the monarchy. The plan was "a thinly disguised attempt to integrate the fiscal institutions of the empire [that] prompted much opposition in the Indies."
The division of contributions envisaged was:
Although the proposal ultimately failed, it was an important factor in the growing mistrust of Castilian hegemony that led to the Catalan Revolt and the Portuguese Restoration War.