Why did Frederick William decide that Brandenburg needed a standing army?

Frederick William, the Elector of Brandenburg, decided to establish a standing army primarily because he recognized that without his own military force, he could never become "master in his own house".1

Upon succeeding his father in 1640, he inherited a land that was ravaged and occupied by foreign troops. By creating a standing army, he sought to secure his authority and combat the privileges of the estates within his individual territories. This military force served as a foundational institution for his increasingly absolutist rule, allowing him to bypass the estates' control over finances and taxation.1

While the army was initially small, it was never entirely disbanded and eventually became the core of the 18th-century Prussian army. Ultimately, the success of this standing army enabled Frederick William to establish Brandenburg-Prussia as a significant European state with a sound fiscal basis and an effective bureaucracy.12