There are many books about Wellington's campaigns during the Peninsular War. However, few examine the critical year 1811, when he advanced in offensive actions and forced Napoleon's armies to retreat over 300 kilometers from the gates of Lisbon to the Spanish borders. For two months, he pursued the retreating French, engaging in conflicts and withdrawals almost throughout the journey.

The French finally stopped at the Spanish borders and attacked Wellington in early May, where an epic three-day battle unfolded at Fuentes de Oñoro. For the rest of the year, Wellington defended the borders while planning schemes for the liberation of Spain in 1811. Wellington's Enlightened Army and the Defense of Portugal examines the famous Army of the Enlightened as it pursued the French and participated in almost every conflict and battle that unfolded that year.

The book also explores the stalemate of January and February 1811, where the formation maintained observatories on high points monitoring the French positions in the region of Santarem, as well as the pursuit of the French army back to Spain in March and April, when the formation participated in many conflicts, battles, and skirmishes, often on its own. These include actions at Pombal, Condeixa, Redinha, Casal Novo, Foz d’Arouce, Freixada, and Sabugal. In May, the Army of the Enlightened participated in a desperate battle at Fuentes de Oñoro, where for much of the battle it held the left wing of the army.

Throughout the rest of the year, the Army was in the region of Ciudad Rodrigo where it held the ground it had held for much of 1810, serving as Wellington's forward observatories. It adopted similar positions and was involved in Fuente Guinaldo and El Bodon. In addition to these battles, the book examines changes in the organization of the formation, with the addition of new battalions and the liberation of other units. It will also analyze the problems it had with command and control—with its exhausted leaders asking for leave to return to their homeland for rest.

Based on diaries, letters, and memoirs, the authors narrate the story of the officers and men who fought in the formation. Many of these sources have never been published before.