 Cádiz had been invested by the French in early 1810 , leaving it accessible from the sea , but in March of the following year a reduction in the besieging army gave its garrison of British and Spanish troops an opportunity to lift the siege . A large Allied strike force was shipped south from Cádiz to Tarifa , and moved to engage the siege lines from the rear . The French , under the command of Marshal Victor , were aware of the Allied movement and redeployed to prepare a trap . Victor placed one division on the road to Cádiz , blocking the Allied line of march , while his two remaining divisions fell on the single Anglo @-@ Portuguese rearguard division under the command of Sir Thomas Graham . 
