Why did French Prime Minister Joseph Caillaux spend two years in prison during World War I?

Joseph Caillaux had served as Prime Minister of France for only seven months. Despite his short tenure of office, he proved to be a hugely controversial figure. In particular, he was strongly criticized for his pro-German foreign policy, which large sections of public and political opinion felt was not in the Republic's best interests. To make matters worse, it emerged that Caillaux had been conducting clandestine negotiations with the German government behind the back of Fallières, president of France. The ensuing scandal forced Caillaux from office.
Yet even after his resignation, Caillaux continued to be a figure of public controversy. When the First World War broke out, he was a leading advocate of a peaceful settlement with Germany. This earned him the undying hostility of many, who accused him of being, at best, a defeatist, and at worst, a traitor. In 1918, Caillaux was arrested and charged with high treason. After a lengthy delay he was sentenced to three years' imprisonment, but with time served he was released. He was also banished from French territory for five years and deprived of his civil rights for ten years.