Often called Suruga Dainagon (the major counsellor of Suruga), Tadanaga was born in 1606. His birth name was Kunichiyo. The date of his birth is uncertain, and is variously given as May 7, June 1, and December 3. Blessed with military and intellectual prowess and a generosity of spirit, he received support from his mother, Oeyo (or Sūgen'in), who favored him over her other son Takechiyo (the future Iemitsu) to become the third shogun. Just before the death of their father shogun Hidetada, Iemitsu accused his brother, already under house arrest in Kofu, of insanity, stripped him of all possessions and offices, leaving him to commit seppuku.