After Wellington's decisive defeat of King Joseph at Vitoria, he advanced to capture San Sebastián and Pamplona, the last French outposts on Spanish soil.
While Wellington concentrated his efforts on capturing the strategically important port of San Sebastián, he sent 11,000 men under the Irish-Spanish General O'Donnell to blockade Pamplona. To prevent a French counter-attack over the Pyrenees Wellington positioned General Hill's Corps over a 50 mile (80 km) front, to cover the coastal road and the main passes over the mountains.
Having rapidly rebuilt and reorganised his forces after their defeat, the French under Marshal Soult launched an attack towards Pamplona through the passes of Maya and Roncesvalles.
The French force at Roncesvalles consisted of 40,000 men and 8 mountain guns under Generals Reille and Clausel. The pass was defended by the British 4th Division commanded by Major-General Galbraith Lowry Cole and was helped by the Portuguese 4/10 brigade
The French attacked from Saint-Jean-Pied-de-Port advancing in two columns either side of the pass. Clausel on the Altobiscar and Reille on the Linduz. At around 06:00 the two armies met. The British were badly outnumbered, but fought steadfastly. Byng's brigade on the eastern side held Clausel's division at bay for three hours before being forced back.
By 14:00 Cole brought up three more brigades as reinforcements. However around 17:00 a dense fog descended over the battlefield, just as the Portuguese 4/10 brigade repelled French attacks in the Val de Baigorry. Cole, despite orders to the contrary, ordered his troops to retreat towards Pamplona.
The Allied forces regrouped and made a stand near the village of Sorauren
Later Wellington admitted that splitting his forces in order to besiege both San Sebastián and Pamplona simultaneously was "one of the greatest faults he ever committed in war".