The capture of San Sebastian on 31 August 1813 left Pamplona as the only Spanish city in French hands. It was surrounded and was being slowly starved into submission.
On 7 October Wellington’s Allied Army of British, Spanish and Portuguese troops invaded France by crossing the River Bidassoa, which was overlooked by the crests of the Pyrenees on the French side. Marshal Nicolas Soult’s French army had tried unsuccessfully to cross the Bidassoa at Irun and Vera on 31 August in an attempt to relieve San Sebastian.
The 1st Division, another British brigade, a Portuguese brigade and two Spanish division forded the river at Irun. The Light Division and three Spanish divisions crossed at Vera, helped by a demonstration by the 6th Division. Unknown to the French, the Bidassoa could be forded near the mouth of the river at low tide. Spanish shrimpers who worked the area led the British 5th Division across three fords at daybreak.
The British divisions all contained Portuguese as well as British troops, except for the 1st Division, which had two British brigades and one of the King’s German Legion, comprised of German expatriates: many of them were from Hanover, whose Elector was also the British King. See Wikipedia for an order of battle.
The 5th Division was across the river before it was spotted by the French. The attack at Irun began when the morning fog lifted at 7:25 am. The 1st Division was half way across the river before it was fired upon. By 11:30 am the Allied objectives in this area had been taken, and Wellington ordered a halt.
Fighting was fiercer at Vera. The Light Division encountered the Star redoubt, but took it after probing for weak spots. It then reached the crest overlooking the river, which were defended by fresh troops, earthworks and artillery. The rapid attack took the crest in several places, and the other defenders retreated.
The Spanish were held up by a hill on the Allied right called the Rhune. However, the French evacuated it the next day in order to avoid being out flanked.
The Allies had crossed the river and quickly taken their objectives. Jac Weller gives Allied casualties as 400 dead and wounded at Irun and 800 at Vera. French casualties were 450 men and all their artillery and most of their baggage at Irun and 1,250 men at Vera. Charles Esdaile says that Allied casualties totalled 1,600 men, half of them Spanish.
Soult fell back to a new defensive line along the River Nivelle.
 J. Weller, Wellington in the Peninsula 1808-1814 (London: Greenhill, 1992), pp. 314-17.
 C. J. Esdaile, The Peninsular War: A New History (London: Allen Lane, 2002), p. 476.