Rubuta ci gaba don wannan sakin layi - Domin a zaɓe shi sarki bayan mutuwar mahaifinsa, Frederick ya ba da babbar tasiri ga masarauta. A matsayinsa na sarki, ya yi yaƙe-yaƙe biyu da Sweden. An ci shi a cikin Yakin Dano-Swedish na 1657<unk>1658, amma ya sami babban shahara lokacin da ya tsallake 1659 Assault on Copenhagen
kuma ya ci nasarar Yakin Dano-Swedish na 1658<unk>1660. Daga baya a wannan shekarar, Frederick ya yi amfani da shahararsa don rushe mulkin mallaka na zaɓaɓɓu don ya goyi bayan mulkin mallaka, wanda ya kasance har zuwa 1848 a Denmark. Ya auri Sophie Amalie na Brunswick-Lüneburg, tare da wanda ya haifi Kirista V na Denmark.