What important school of composers did Mily Balakirev, César Cui, Alexander Borodin, Nikolai Rimsky-Korsakov, and Modest Mussorgsky form?

The Five, a branch of the romantic nationalist movement in Russia

Explanation:

The Five, also known as "The Great Handful", refers to a circle of composers who met in St. Petersburg, Russia, in the years 1856-1870: Mili Balakyrev, César Cuí, Modest Músorgski, Nikolái Rimski-Kórsakov and Aleksandr Borodin. The group aimed to produce a specific type of music from Russia, rather than one that imitated the style on which European music was based, or the training that took place in the conservatories of Europe. In a sense, they were a branch of the romantic nationalist movement in Russia, with the Abramtsevo Colony and the Russian Renaissance, striving to achieve similar goals in the field of fine arts.