As a boy he served in an ambulance corps during the Franco-Prussian War of 1870, and later passed with distinction through the Ecole Polytechnique in mining, becoming a mining engineer, but soon abandoning practical work for teaching, first at Caen and later in the university of Paris.
Towards the close of the Franco-Prussian War he made an admirable defence of Brest, and his organization of the French expedition to the island of St Paul to observe the transit of Venus in 1874 obtained his election to the Academy of Sciences and his promotion as commander of the Legion of Honour.
In the Franco-Prussian War Saarbrucken was seized by the French on the 2nd of August 1870, but the first German victory on the heights of Spicheren, 3 m.
The growth of Clerical influence in France engendered a belief that Italy would soon have to defend with the sword her newly-won unity, while the tremendous lesson of the Franco-Prussian War convinced the military authorities of the need for thorough military reform.
When the Franco-Prussian War broke out, Gabriel Monod, with his cousins, Alfred and Sarah Monod, organized an ambulance with which he followed the whole campaign, from Sedan to Mans.
After the Franco-Prussian War Lavisse studied the development of Prussia and wrote Etude sur l'une des origines de la monarchie prussienne, ou la Marche de Brandebourg sous la dynastie ascanienne, which was his thesis for his doctor's degree in 1875, and Etudes sur l'histoire de la Prusse (1879).
But before the year had closed the events of the Franco-Prussian War compelled his removal to Gottingen.
The sympathy of Rumania with France in the Franco-Prussian War of 1870, and the consequent interruption of certain commercial undertakings, led to a hostile movement against Prince Charles, which, being fostered by Russia, made him resolve to abdicate; and it was with difficulty that he was persuaded to remain.