Complex sentence: Finally, Romefeller and Earth began to re-unite under Relena's influence, but when his old friend Milliardo was leading the White Fang in a war against Earth, Treize seized the opportunity and once again took control, this time of the newly formed World Nation.
Split into simpler sentences: Finally, Romefeller and Earth began to re-unite under Relena's influence. When his old friend Milliardo declared war against Earth, Treize seized the opportunity to take control of the newly formed World Nation.