Benito de San Juan was a Spanish military officer and a notable commander of the Spanish forces during the Peninsular War. He was the commanding officer of all the Spanish forces during the Battle of Somosierra.
Benito de San Juan started his military career as a Lieutenant Colonel of Hussars of Estremadura during the War of the Oranges. Noted by Manuel Godoy, he was promoted to the rank of Colonel and in 1802 he became the commander of Godoy's personal guard. Soon afterwards he was again promoted, this time to the rank of Brigadier. In 1805 promoted to the rank of mariscal (in fact a general's grade), he became the general inspector of Spanish infantry and line cavalry.
During the Peninsular War he assumed the command over the major part of the Spanish forces defending Madrid. Knowing the weakness of his forces, much inferior to the French army which was considered the best fighting force of its times, he prepared a plan of indirect defence of the Spanish capital by defending the Somosierra pass on the road leading towards it. However, in the effect of the battle of Somosierra he lost most of his artillery and his force was dispersed, while de San Juan himself was wounded several times during the charge of the Polish cavalry.
He managed to withdraw with the remnants of his routed forces to Talavera, where he attempted to rally his army. However, the demoralisation of the Spanish army, composed in a large part of untrained conscripts and peasant volunteers, led to a mutiny. On January 7, 1809 the mutineers captured their general and executed him.