Two weeks later, a new teacher of rhetoric, the 22-year-old Georges Izambard, started at the Collège de Charleville. Izambard became Rimbaud's mentor, and soon a close friendship formed between teacher and student, with Rimbaud seeing Izambard as a kind of elder brother. At the age of 15, Rimbaud was showing maturity as a poet; the first poem he showed Izambard, "Ophélie", would later be included in anthologies, and is often regarded as one of Rimbaud's three or four best poems. On 4 May 1870, Rimbaud's mother wrote to Izambard to complain that he had given Rimbaud Victor Hugo's Les Misérables to read.
The Franco-Prussian War, between Napoleon III's Second French Empire and the Kingdom of Prussia, broke out on 19 July 1870. Five days later, Izambard left Charleville for the summer to stay with his three aunts – the Misses Gindre – in Douai. In the meantime, preparations for war continued and the Collège de Charleville became a military hospital. By the end of August, with the countryside in turmoil, Rimbaud was bored and restless. In search of adventure he ran away by train to Paris without funds for his ticket. On arrival at the Gare du Nord, he was arrested and locked up in Mazas Prison to await trial for fare evasion and vagrancy. On 5 September, Rimbaud wrote a desperate letter to Izambard, who arranged with the prison governor that Rimbaud be released into his care. As hostilities were continuing, he stayed with the Misses Gindre in Douai until he could be returned to Charleville. Izambard finally handed Rimbaud over to Mme Rimbaud on 27 September 1870 (his mother reportedly slapped him in the face and admonished Izambard), but he was at home for only ten days before running away again.