His opponents learned from Napoleon's innovations. The increased importance of artillery after 1807 stemmed from his creation of a highly mobile artillery force, the growth in artillery numbers, and changes in artillery practices. As a result of these factors, Napoleon, rather than relying on infantry to wear away the enemy's defenses, now could use massed artillery as a spearhead to pound a break in the enemy's line that was then exploited by supporting infantry and cavalry. McConachy rejects the alternative theory that growing reliance on artillery by the French army beginning in 1807 was an outgrowth of the declining quality of the French infantry and, later, France's inferiority in cavalry numbers. Weapons and other kinds of military technology remained largely static through the Revolutionary and Napoleonic eras, but 18th-century operational mobility underwent significant change.
Try to answer this question if possible (otherwise reply "unanswerable"):  Napoleon's use of artillery led to its increased importance after what year?
1807