The Battle of Somosierra took place on 30 November 1808, during the Peninsular War, when a combined Franco-Spanish-Polish force under the direct command of Napoleon Bonaparte forced a passage through a Spanish Division stationed at the Sierra de Guadarrama, which shielded Madrid from direct French attack. At the Somosierra mountain pass, 60 miles (97 km) north of Madrid, a heavily outnumbered Spanish detachment of regulars, volunteers and artillery under Benito de San Juan aimed to block Napoleon's advance on the Spanish capital. Napoleon overwhelmed the Spanish positions in a combined arms attack, sending the Polish Chevau-légers of the Imperial Guard at the Spanish guns while French infantry advanced up the slopes. The victory removed the last obstacle barring the road to Madrid, which fell several days later.

The Peninsular War (1807–1814) was the military conflict fought in the Iberian Peninsula by Spain, Portugal, and the United Kingdom against the invading and occupying forces of the First French Empire during the Napoleonic Wars. In Spain, it is considered to overlap with the Spanish War of Independence.