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.

[5] 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.

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.)

[6] A professional officer of considerable talent (albeit new to command),[7] 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.

[11] Bessières, well-informed of the Spanish plans by virtue of a double-agent, advanced from Burgos 9 July in the aim of preventing Blake's junction with Cuesta, resolving to concentrate his effectives en route.

Receiving part of a division at Palencia on 10 July, Bessières rapidly assembled 14,000 with 40 guns and marched to meet Blake and Cuesta, approaching the Spanish positions along the cultivated plains of Medina de Rio Seco at dawn on the 14th.

[8] Bessières immediately understood his enemies' weakness and moved to seize the central position, allowing him to dispatch the two Spanish wings in detail by keeping Cuesta at bay with a screening force (Major-General Mouton) while elements of two divisions stormed the ridge under his supervision.

Bessières' cavalry reserves then charged into the gap kept open by Mouton and tore into Blake's right flank, cracking his fragile force and driving it off the ridge in a panicked rout.

[3] Before Bessières could turn on Cuesta, the Spanish general, quite unwilling to follow Blake in retreat, formed his troops into columns and hurled them uphill at the Imperial army, now drawn up on the ridge.

The leading Spanish grenadier battalions struck their last determined blows against the French centre before being caught in this crossfire and brusquely forced off the ridge, convincing Cuesta to sound the retreat.

Cuesta's divisions (though not the commander himself) received special praise for nearly securing a dramatic victory even after Blake's rout: The battle of Rio Seco, though unfortunate, was far from dishonourable to Spanish prowess...

"[12] British military historian David G. Chandler pinned the blame for the defeat squarely on Cuesta, who for reasons not quite clear to him refused to deploy his portion of the army against the enemy and planted his divisions far to the rear.

Likewise, according to Foy, the Spanish deployment did not offer much prospect for success: approaching a prepared enemy frontally along the defile, with both flanks open to attack, and with such a gap between the two lines, all but guaranteed defeat.