The Kowa Period can be considering as both Zero Requiem's goal and result and is described as a "Miraculous Tomorow" by the later generations.
After Suzaku (dressed as Zero) killed Lelouch, most of the hatred died with him, and the world that joined hands to defeat him continued to wish for peace and chose the cooperation route even after Lelouch’s death, just like he planed.
A lot of major political changes came with it, beginning with the fall of the three superpowers that once ruled over the world: the Chinese Federation split apart and the United Republic of Europia lost most of its territories and all its international influence during R2, while the Holy Britannian Empire lost most of its culture's components and colonies and has been downgraded to a democratric principality. But at the same time, the UFN, after regaining its legitimacy and being reorganized, united more than 80% of the world (more than all the other superpowers), and is now described as the center of the world. This lead to an increase of the democracy around the world, but also to the transfer and use of the members' military power to the Black Knights (as delegated by the United States Charter ).
A new calendar system (simply named the Kowa Period), which starts at Lelouch's death, has also been adopted.
Thanks to it, no large-scale conflict ever occurred, and in the second year of the Kowa Period, at the beginning of Code Geass: Lelouch of the Re;surrection, an entire year without war or terrorism has just passed.