The French planning for (what would be) WW1 started in the period immediately following France's massive, humiliating defeat in the Franco Prussian War.

The early plans (Plan I - XII, roughly the end of the FPW to the end of the 1880s) were defensive in nature - focusing almost entirely on the mobilisation of troops to allow for the defence of France's border with Germany.

In the 1890s, two big changes occured - 1) France passed new conscription laws (1889) that resulted in an increase in the size of the army; and 2) the Dual Alliance (between France and Russia) began to take shape, before being secretly signed in 1894.

As a result, the French began to adopt a "defensive-offensive" strategy - the defense of the border (along the line from Reims to Belfort), coupled with a large strategic reserve that would counterattack once the initial German attack was repulsed.

In 1904, a German officer (referred to as Le Vengeur) sold a copy of the German mobilisation plans to the French, which provided evidence that the Germans would try and outflank the French through Belgium and Luxembourg. As a result of this, coupled with further changes to France's conscription laws (1905) led to the creation of new plans - Plan XV, adopted in 1906, and later XVI (1909).

XV and (to a greater extent) XVI envisaged a defensive line against the German and Belgium borders, with a reserve army at Verdun to counterattack where possible. A preemptive attack into Belgium was ruled out for political reasons, so the plan still called for a defensive-offensive operation: wait for the German attack (which would violate Belgium's neutrality and hopefully bring the UK into the war), and then repulse them using the reserve army.

It was not until 1914, with the adoption of Plan XVII, that the idea of a defensive-offensive operation was dropped. Instead, XVII proposed a concentration of forces further east than XVI, allowing for immediate counter attacks against German advances through Belgium, Luxembourg or into France proper. The idea was not to drive to Berlin, but to launch offensive operations that would blunt any German attacks before they could get underway. Once that was achieved, France and Russia would be able to invade Germany simultaneously.