The Battle of Medina de Rioseco, also known as the Battle of Moclín, was fought during the Peninsular War on 14 July 1808 when a combined body of Spanish militia and regulars moved to rupture the French line of communications to Madrid. General Joaquín Blake's Army of Galicia, under joint command with General Gregorio de la Cuesta, was routed by Marshal Bessières after a badly coordinated but stubborn fight against the French corps north of Valladolid.
Bessières exploited the poor coordination between Blake and Cuesta to defeat the Spaniards in detail, with Blake being ejected from a low ridge while Cuesta sat to the rear, and Cuesta failing to recapture the ridge with his own troops. The Army of Galicia was the only formation capable of threatening the French advance into Old Castile—Cuesta's command having been destroyed earlier at Cabezón—and its destruction marked a serious blow to Spain's national uprising.
But in the event, Medina de Rioseco proved to be the solitary French triumph in an invasion of Spain that ultimately failed to seize the country's major cities or to pacify its rebellious provinces, and which met outright disaster at Bailén, forcing French forces—Bessières' victorious corps included—to fly over the Ebro in retreat. A fresh campaign, conducted by Napoleon himself with the bulk of the Grande Armée, would be needed to redress the situation.
Background
The Spanish conventional warfare had started with the Battles of El Bruch.
Situation in northern Spain
Recent French operations in the region had come far short of Napoleon's expectations. The Galician and Biscayan provinces were ideally suited as a base for resistance against France: remote and mountainous; out of the French Army's immediate reach yet flanking its long communications to occupied Madrid; its coastline largely secured by the allied Royal Navy, which disgorged supplies and materiel. In June, Marshal Bessières' flying column, marching on Santander in an attempt to secure French communications in Galicia and guard the coast against a possible British landing, had been forced back by popular resistance. Stung by these and other reverses, Napoleon committed more troops and formulated a new strategy. In July he ordered Bessières to renew his western offensive.
Snaking toward the French were the columns of the Army of Galicia under General Joaquín Blake who supplemented his force with Cuesta's motley levy of militia and regulars from isolated provincial garrisons—debris from Cuesta's destroyed Army of Old Castile.
Spanish preparations
Cuesta, quite undeterred by his defeat the previous month, proposed a rapid coup de main toward Valladolid, astride the French communications (and incidentally his old seat of command as Captain General of Old Castile, from which he had been ejected after his rout at Cabezón.) Cuesta mustered some 350 horse—a precious, if token, force, considering Galicia and Asturias combined could not furnish as many—and several infantry battalions, but not a single cannon. The northern Juntas received Cuesta's proposals coolly; Asturias refused to be drawn into what it considered a foolhardy scheme but committed several battalions in a show of goodwill; Galicia, however, dispatched General Blake to liaison with Cuesta. A professional officer of considerable talent (albeit new to command), Blake questioned the wisdom of facing the Grande Armée in open country, preferring the broken ground and hills of the north to neutralize the superiority of French arms. Of particular concern to Blake was the dilapidated Spanish cavalry, with which a descent into the plains of Castile seemed a sorry prospect. The Galician commander advocated holding and fortifying the rugged terrain of Léon and Galicia, but deferred to Cuesta.
Between them the two Spanish generals raised about 25,000 men, many of them dispirited and in poor condition. In May British ships had disembarked some 5,000 former Spanish prisoners of war (largely captured in attacks on the Spanish colonial empire during the recent Anglo-Spanish War) with arms and munitions, notably Cuesta's 800 colonial regulars—the Colorados battalion—taken prisoner in Montevideo and, for lack of Spanish uniforms, bedecked in full redcoats. The Colorados de Buenos Aires had been taken prisoner during the unsuccessful British attacks on the Viceroyalty of the Río de la Plata. Cuesta, citing his seniority, claimed supreme command and set his columns marching on 12 July, against Blake's objections (although the Castilian was persuaded to detach a reserve at Benavente). Cuesta, for lack of cavalry, advanced blind to French movements, expecting to find Bessières concentrating near Valladolid.