The Kowa Period can be considering as both Zero Requiem's goal and result and is described as a "Miraculous Tomorow" for the later generations.
After that Suzaku (dressed as Zero) has killed Lelouch, most of the hatred died with him and the world that joined hands to defeat him continues to wish for peace and choose cooperation route even after Lelouch’s death, just like he planed.
A lot of major political changes came with it, started with the fall of the three superpowers who once ruled over the world ( the Chinese Federation has 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 in the same time, the UFN, after has regained its legitimacy and been reorganized reunites more than 80% of the world ( it's more than all the other superpowers) and is now discribed as the center of the world. This lead to a increase of the democracy to the world, but too 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), who starts at Lelouch's death, has also been adopted.
Thank 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.