Short answers, pulled from the story.
Napoleon recaptured the city of Reims from Allied forces after a surprise attack. French troops under Ney and Friant arrived to support the emperor's Guard cavalry and infantry divisions which forced the Russian line back toward Laon.
Count Alexander Ivanovich Saint-Priest led the Allied army that defended Reims against Napoleon. The Allied columns included units from Russia such as the Kiev Dragoons and Prussia including the 3rd Pommeranian Landwehr Regiment.
The battle took place on the 13th of March 1814 when Napoleon ordered an immediate attack by 4:00 pm. French forces successfully drove the Allies out of the city and pursued them toward Laon.
Historian Digby Smith recorded approximately 900 French casualties while Allied losses reached about 2,700 men and 22 guns combined. George Nafziger listed Allied losses as between 700 and 800 dead plus 1,500 wounded and thousands captured.
Napoleon attacked to defeat Count Saint-Priest near Reims and reoccupy the town after learning it had been captured. The emperor sought a cheap victory to counter the Allied advance following his earlier defeat at Laon.