Ha, and you probably thought you'd make it to the end of the year without any more history books concerning the Europe in the first half of the 20th Century. Silly audience.
The Maginot Line is perhaps the most famous futile defensive military structure in history, as the French built this massive stretch of fortifications along their border with Germany, only to see them thwarted when the Germans unleashed the brilliant strategy of. . . going around it. Hughes' work is an attempt to explain why France went that route, and how a series of decisions and circumstances gradually boxed them in, until this was the only decision they seemed to have left.
A lot of it boils down to demographics, that France was at a severe numerical deficit after World War 1, and was painfully aware of it. They came out of the war with a smaller population than they went into it with, and Germany (which already had more people) did not. And the biggest hit was to the French citizens who would have produced the next generation of soldiers, so they problem was only going to increase. Combined with France's inferiority to Germany in industrial capacity (even if Germany was prohibited from utilizing this capacity for war), and France felt very threatened, but also didn't really want to fight again if it could be avoided. So they want to keep Germany weak militarily, but try to do so in ways that don't make them seem to be the aggressor, because they don't want to lose Britain's support. They try to form alliances with the new countries on Germany's eastern border, but not the sort of alliances that would force them to fight if Germany invades those countries. They start out trying to keep the Rhineland demilitarized, and establish outposts there, with the idea that in that way they can keep any fighting in Germany, and spare France a repeat of the past war.
It becomes a series of steps back and compromises that steadily weaken their position, which they are aware of, and that forces them into an ever more defensive posture, which makes them feel weaker, and so on. The number of troops they station near the Rhineland keeps reducing, as does the term of service, in part because they can't foot the bill to maintain a larger army. They pay officers so poorly that several of them won't accept appointments to their military colleges, because they can't afford to live in Paris and attend. Which hurts the readiness of the army, which causes France to feel that much less confident about their ability to resist Germany. They gradually stop thinking about responding to German aggression by advancing into Germany and fighting there, and more about building defenses to supplement their manpower. Even in the early going, the defenseworks were meant to support the army, to increase its maneuverability, but the weaker France's army got, the less that became an issue.
And so, if the military couldn't protect the country, maybe concrete would. You could say they ignored the fact Germany might well attack through Belgium, as they did last time, but Hughes points out the French knew this very well. But the Belgians were not eager to tightly ally with France, especially when the end result seemed to be France and Germany fighting a big war within Belgium's borders. At the same time, France didn't want to simply build a bunch of defenses on their border with Belgium, because that wouldn't really encourage an alliance, to wall the ally off outside with the enemy (also, the part of France apparently doesn't have the sort of topography that encourages defenses). So they were going to concentrate the military there, but still wanted to feel secure along their common border with Germany.
Hughes makes a lot of good points, thought she basically stops once you get to the 1930s. Strangely though, the way almost everything ties together so neatly felt off. It was almost too obvious and smooth a path to where things wound up. I would have expected more fits and starts, the French heading in one direction, then changing mid-stream and going a different way in some half-assed manner. But the way Hughes presents it, it's a steady but gradual thing where France keeps making these decisions and one decision - reducing service from 2 years, to 18 months, to eventually 1 year - undermines the size of the force they have ready at any given time, which makes them concerned about whether they can afford to let the standing army go on the offensive, because they have to buy time to mobilize everyone else, and so they start trying to counteract that with structures to slow the enemy, tacitly admitting they've abandoned the idea of taking the fight onto German soil. It feels very logical, but I guess I'm not used to governments proceeding logically over a period of several years. Especially one that was turning over the people in charge as frequently as the Third Republic (even if it was really just recycling the same guys over and over again).
'In coming to depend upon such works France would tacitly relinquish its option to reoccupy German soil and would simultaneously abandon its interests in eastern Europe. And in thus implicitly renouncing the one substantial provision of the Locarno agreements, the French would feel a still greater need to buy time, in this case, in order to avoid a war they could not win.'