Emperor Sujin (10th emperor) is also known as Hatsukunishirasu Sumeramikoto and this name (literally meaning the "first emperor who ruled the country") implies that he was the first emperor to govern the country.
第10代崇神天皇は別名ハツクニシラススメラミコトといい、すなわち初めて天下を治めた天皇であることを名称で物語っている。