After a stroke in 1857 the king was partially paralyzed and largely mentally weak , therefore his brother William became regent from 1858 until the king 's death in 1861 , then he became king himself as William I. A stroke in 1857 left the king partially paralyzed and largely mentally incapacitated , and his brother William served as regent from 1858 until the king 's death in 1861 , at which point he acceded the throne himself as William I.