What was the battle of Verdun, when did it occur, who was involved, and why?

The Battle of Verdun in 1916 was the longest single battle of World War One the casualties from verdun and the impact the battle had on the French Army was a primary reason for the British starting the battle of the somme in July 1916 in an effort to take german pressure off of the French at verdun. The Battle of Verdun started on February 21st 1916 and ended on December 16th in 1916. It was to make General Philippe pétain a hero in France.