During the 1870s, the French army drew up concentration plans according to a defensive strategy, which exploited the Meuse and branches of the Moselle parallel to the 1871 border. The completion of the fortress lines between Belfort and Verdun in the late 1880s and railway building from the interior to the border, then gave the French army the means to contemplate a defensive-offensive strategy, in which a German attack would be repulsed and then followed up by a counter-attack. In August 1891, Plan XI was completed, with an option for an offensive as well as a defensive strategy from the start, to exploit the opportunity created by the improvement in relations between the Third Republic and the Russian Empire. The Franco-Russian Alliance led to Plan XII in February 1892, in which an immediate invasion of Germany was considered possible but from Plan XI to Plan XVI, the strategy remained defensive-offensive, French attacks being expected after the repulse of a German invasion.