What were the Lines of Ne Plus Ultra in the War of the Spanish Succession?

Question

After reading another biography of Marlborough, I am annoyed by the fact the authors mention but do not describe the Lines of Ne Plus Ultra. They make it sound (I have read Chandler and Konstam) as if it was the ultimate system to defend France, and yet Marlborough 'defeated'/crossed it in a matter of weeks.

Admittedly, Marlborough was skilled but... it doesn't make sense based on the 'ultimate' level of defense these lines are described to be in these biographies (without providing detail). English Wikipedia has nothing at all on this. So, what do we actually know about the Lines of Ne Plus Ultra—what were they and what was their intended function?

Note: I imagine I am held back by not searching in French, but I am not 100% convinced.

Answer 1

In one sense, it might be better to say that Marlborough "bypassed", rather than "defeated", the Lines of Ne Plus Ultra. The following quote is from the 2011 biography of Marlborough by Angus Konstam:

The French had built a defensive line, named Ne Plus Ultra (No Farther). To Marlborough this was the perfect challenge. It ran from Arras to Cambrai and Valenciennes, then linked up with the existing defensive lines at Sambre.

In the end, Marlborough breached these lines whilst losing almost no casualties. Marlborough feinted elsewhere to distract Marshal Villars, and then on 6 July [1711] stormed and captured Arleux near Arras. Marlborough retired again, and the French retook the town, but the defences had been razed. In effect it created a breach in the Ne Plus Ultra line. Marlborough moved to the west to the countryside around Denain, and Villars followed. At that point, Marlborough doubled back and crossed the lines at Arleux, which was virtually undefended. It was a magnificent piece of manoeuvring.

It goes to show that it doesn't matter how impressive the defensive structures may be, if they are not effectively manned they will be ineffectual.

You can find a more detailed description of the Ne Plus Ultra line in Chapter 18 of Marlborough: The Hero of Blenheim by John Hussey (2015). In brief,

These Lines ran from the sea to the Ardennes, a total distance of some 160 miles, being a combination of defended riverbank, marsh, earthworks, formal town defences and individual forts. Garrisons held certain fixed points and could be supported by a mobile army which would shadow the movements of any attacking force.

Hussey includes a lot more detail.

Answer 2

The Ne PluS Ultra line was based on a system of fortresses built by Vauban. Its claim to fame was that it "extended" from the English Channel to the Swiss border, but it was only about two (fortresses) deep.

The line was not a cure-fall for the French defensive system. Basically, the fortresses were the hinges of the "line," which was only as good as the ability of the French to cover the gaps between the fortresses. Another answer discussed how Marlborough was able to "pick off" an underdefended fortress at Ariex, near Arras. When the French army moved to repair this breach, Marlborough went for his real objective, the strongest fortress of Bouchain, at the heart of the line, and on the direct road to Paris.

Both sides had about 90,000 men, but Marlborough detached 30,000 to conduct the siege (against 5,000 defenders), so his covering force of 60,000 was outnnumbered 3 to 2 by the French. Not only did he defeat attempts at relief, but he strengthened his supply line going back to Belgium. When the siege was over, there was only one fortress between Marlborough and Paris. This victory is less heralded than others because it involved less than 10,000 French casualties, but Winston Churchill (a descendant of Marlborough's), considered it his "finest" victory, because it was won at a location, and on terms and conditions of the French choice.

Like the Maginot Line, the Ne Plus Ultra Line was "porous" because the British army outmarched the French, first at Arieux, then at Bouchain. (Wellington won battles with similar tactics in Spain.) In "A War to be Won" Professor Williamson Murray attributes the German victory in France in 1940 to the fact that the German Army (and armor) went on "long training marches" in the winter of 1939-40 and the French did not. Basically, Marlborough's penetration of the Ne Plus Ultra Line was an "attack through the Ardennes," so it's unfortunate that it is so poorly understood.