Guerrillas
Wellington greatly feared an outbreak of guerrilla warfare against his soldiers by the French people. He was mindful that after the Battle of Nivelle, Spanish troops had indulged in plundering French civilians. Accordingly, he took pains to send most of his Spanish units back to their home country, rightly concerned that they would inflict the same atrocities on French civilians that their own had suffered from Napoleon's armies. At first, only Pablo Morillo's division was brought forward because it depended on the British payroll and supply train. As in Spain, Wellington harshly punished any British soldiers caught marauding.
As it happened, the French troops, hardened by years of plundering foreign civilians, began to despoil their own citizens. Actually, despite their French citizenship, the inhabitants south and east of Bayonne were Basque people for the most part, often unable to speak and/or understand French as it had been the case before crossing the River Bidasoa. Soult tried to crack down on looting, hanging offenders and even shooting a much-decorated captain, but he was unable to stop it. As one historian noted, "The civilians soon came to the conclusion that only a swift allied victory could save their countryside." Consequently, Wellington had little to fear from French guerrilla action.
Read more about this topic: Battle Of The Nive