In the summer of 1809, the British army of Arthur Wellesley marched into western Spain to join Gregorio García de la Cuesta's Spanish army. Wilson's 3,500-man Portuguese-Spanish force served as the left flank guard of this offensive. The Allied armies defeated King Joseph Bonaparte's Imperial French army at Talavera at the end of July. However, the threat of Marshal Nicolas Soult's large army to the north soon forced Wellesley and Cuesta to withdraw to the west.