Frontière de fer or pré carré is the name given in military historiography to the double line of fortresses that king Louis XIV of France had constructed after the Peace of Nijmegen in 1678 to protect what was then Northern France against foreign invasion, and to be used as operational bases against foreign enemies in the years of the Nine Years' War and the War of the Spanish Succession.

[1] The term pré carré was first used by the French military engineer Sébastien Le Prestre de Vauban in a letter to the French minister of war François Michel Le Tellier de Louvois of January 1643 in which he wrote:[2] Seriously, Monseigneur, the king should think a little about making his "own garden" (pré carré).

[a]In this quote Vauban not only introduced the term pré carré that can be variously translated, depending on the context,[b] but more importantly, for the first time formulated the strategic vision that would later be the basis for the frontière de fer as a military architecture.

Though Vauban can claim to be the originator of the concept, others (primarily Louvois, and possibly the king himself) decided the final shape the system would take over many years.

According to the principles he developed in his Mémoire, pour servir d'instruction dans la conduite des sièges et dans la défense des places (1670) and worked out in many later such mémoires, he sought to adapt each individual design to the characteristics of the local landscapes, taking advantage of local features, where possible.

To construct such a position Vauban sought to take advantage of natural obstacles, like rivers, canals and military inundations.

It consisted of a "first" or "northern" line of fortified places, centered on Mons, Condé and Tournai, with in front of that the fortress of Ath.

Knokke, Ypres, Menen, Tournai, Mons and Namur so became part of the defensive line of fortresses that protected France's enemies.