The 1807-1814 war in the Iberian Peninsula was one of the most significant and influential campaigns of the Napoleonic Wars. Arising from Napoleon's strategic necessity to impose his rule over Portugal and Spain, it evolved into a constant drain on his resources. Sir Charles Oman's seven volume history of the campaign is an unrivalled and essential work. His extensive use and analysis of French, Spanish, Portuguese and British participants' accounts and archival material, together with his own inspection of the battlefields, provides a comprehensive and balanced account of this most important episode in Napoleonic military history. Volume IV covers the period during which Portugal was finally secured from the danger of French conquest. French successes in Spain continued, but the army under Massena was forced finally to retreat from Portugal. The Allied offensive began to gather momentum, ' although their attempt to recapture Badajoz was unsuccessful.