Napoleon's decision to attack the advancing Allied force was to seize initiative from a not yet unified invasion.
This force consisted of Blücher’s Army of Silesia – Bernadotte’s Army of the North and Schwarzenberg’s Army of Bohemia.
A strung-out Allied army under Blücher quickly threatened Paris itself, and Napoleon's French army looked to seize the moment to destroy Blücher’s Army of Silesia. They succeeded in exploiting Blücher’s carelessness, but were eventually defeated by overwhelming odds in the theater as a whole.
Napoleon systematically split the Army of Silesia, and attempted to isolate their forces to provide himself with superior numbers over the course of smaller battles.
Being the furthest East, Blücher looked to quickly crush the French army. He receives intelligence that Napoleon was traveling northeast rather than east and changes the marching orders of his corps, causing a slight delay. Kleist and Kapzevitsch march southwest supposedly towards Napoleon and Sacken carries on the pursuit of MacDonald so Yorck can march south to eventually envelop Napoleon from the west.
In reality, Napoleon is marching towards Olssufiev – and as expediently as possible.
Napoleon and Olssufiev happened to collide at Champaubert, where Napoleon's forces crush Olssufiev.
Olssufiev’s corps being annihilated, Napoleon’s striking force occupies the central position between two pairs of Allied corps. Napoleon breaks his force down into three major parts: Marmont commands a relatively small force to monitor Kleist and Kapzevitsch’s progress while Nansouty and Mortier command roughly equal forces to confront Yorck and Sacken.