On the Ruin of Britain by Gildas is an influential historical text that details the decline of Britain in the post-Roman era. Written by the British monk Gildas in the 6th century AD, the book offers a firsthand account of the political, social, and religious landscape of the time, and offers a scathing critique of the leaders and society of Britain at that time.

The book is structured as an open letter to several British leaders, including Constantine of Dumnonia, Aurelius Conanus, and Vortiporius, and begins with an overview of the history of Britain up to the author's own time. Gildas then describes a series of moral failings that he believed were responsible for Britain's decline, including greed, corruption, and a lack of Christian piety. He also condemns the British leaders of his time for their incompetence and unwillingness to defend their country against outside invaders, such as the Saxons.

On the Ruin of Britain is an important primary source for scholars studying the early history of Britain. It provides a rare glimpse into the cultural and political climate of the time, and offers insight into the mindset of the people who lived through this tumultuous period. The book is also notable for its vivid descriptions of the battles and skirmishes that took place between the various British factions, and for its critique of the Church's role in society.

Despite being written over a millennium ago, On the Ruin of Britain remains relevant today, as its themes of political corruption and societal decline continue to resonate with modern readers. It has been translated into many languages and has influenced the work of countless historians and scholars over the centuries.

In summary, On the Ruin of Britain by Gildas is a significant historical text that provides valuable insight into the political, social, and religious climate of Britain in the 6th century AD. Its enduring relevance and continued impact on the study of British history make it a must-read for anyone interested in the subject.