What about situations where the defense is stronger, and the enemy will exhaust himself attacking you?
I think such situations are very rare.
Usually when you are stronger, you want to attack rather than defend.
Defence is by definition, usually - with some exceptions of course - applied by the less numerous or / and the weaker one.
And as Flying Pig wrote, attack surely requires to have firepower superiority - but it can be done by applying the concentration and economy of force rules, even if along entire frontline you don't have a 3 to 1 superiority in firepower, you can concentrate most of your forces in a key area to achieve it.
Flying Pig, you also wrote that aggression and speed are necessary for a successful attack. Thanks for ensuring me in my idea about this!
It seems that such thing like
"slow, systematic, methodical attack" is a very bad idea. Because it gives time for defender to bring in reinforcements.
Such attacks, preeceded by hours and hours or sometimes days of artillery preparation, were being carried out in WW1 against enemy trenches...
French war doctrine preferred such systematic, methodical attacks during the 1920s, 1930s and in 1940 - as the result all of their counterattacks in 1940 campaign simply failed. On the other hand, Nazi Germany's mode of warfare - so called "Blitzkrieg" - relied on speed, aggression and boldness of actions.
Surprise effect and confusion in enemy lines caused by it are always better than many hours of artillery preparation I suppose.
But in Pacific islands campaigns the US successfully relied on their many times superior firepower to overcome Japanese dug-in defences.
It is much better though, to deliver a vast volume of firepower over a short period of time, rather than bombarding enemy positions for many days.
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BTW - is there any use for steel-and-concrete fortifications nowadays? For example would something like a Maginot Line be of any use nowadays?
It seems to me that after WW2 there were no any major military conflicts where such fortifications played important role. But maybe I'm wrong?