People seek security in an uncertain, not exactly safe world. The consequences can be self-defeating.
One such example is an arms race. One state is a bit superior to his neighbour - the neighbour can't stand it and strives for superiority as well. Thus they spiral into an arms race, in worst case till they go beyond the sustainable strength. Inevitably one power realises that its strength isn't sustainable - but probably doesn't dare to accept inferiority. The result may be a war - the attempt to exploit a fleeting moment of unsustainable superiority that promises a better end than inferiority.
The situation in 1912-1914 and especially after the first mobilizations in 1914 was similar.
A theoretical way out is to build up military strength that's not of use in strategic offence. This way one power could be at strength "200" if defending itself and "100" if it's an aggressor - with a similar neighbour. Neither power would need to fear inferiority in war and no arms race would be necessary to feel safe.
Fortifications used to provide exactly this; military utility in strategic defence that's not directly useful for strategic offence. Permanent fortifications have lost their strategic utility, though. Even the fortifications along the Korean border don't come close.
Medieval fortresses are popular among tourists and generally capture the attention and imagination of people. These visitors don't reall understand how bad it really was when fortifications lost their strategic relevance.
Burg Hochosterwitz, photo: Johann Jaritz
There's a general problem on the tactical level that makes it difficult to buy exclusively "defensive" weapons (even anti-tank missiles have offensive uses!) for modern ground warfare: Operational defence neccessitates offensive actions on the tactical level. In fact, even stationary defensive concepts don't work without tactical counter-attacks.
Battlefield air defences are probably the only exclusively defensive combat component of ground forces and even they are of greater use during mobile (potentially offensive) operations than during static phases (= never offensive).
A competent operational defence with mobile forces is usually offensive on the tactical level, too: It's mostly about giving some ground in exchange for a good opportunity to counter-attack.
The power of mobile combined arms operations disqualifies the concept of "slow" (and therefore not much "offensive") ground forces such as infantry-centric forces. A total defence concept is not worth its budget if it cannot withstand a smaller and balanced offensive concept.
Finally, strategic defence is rarely exclusively defensive and successful at once. Successful examples that came close to total defence depended very much on geographic obstacles.
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For clarification; defence is supposed to be the superior form of combat on the tactical level, but only with an at least similar degree of combat readiness. Real warfare tends to be tricky in this regard.
Defence is strong, but it's not decisive. History knows many example of two forces of about equal total strength clashing and the defensive one lost. This tragedy has hit many army commanders on the field of battle and modern European warfare had also several extremely impressive examples.
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The only realm where a total defence concept seems to have the potential of success is the political level. The invention of defensive alliances (where members are not supposed to support an aggression of an ally) was ground-breaking.
The Cold War has taught us that this doesn't suffice to avert wasteful arms races. The problem was that NATO was perceived as one bloc by the Kremlin.
The Kremlin did indeed believe that all important NATO members would participate in a hypothetical aggression, even though they would not have been legally obliged to.
The post-Cold War NATO shows an even worse side-effect: Bored by the peace and relative lack of tensions among members, their attention became focused outward on minimal to ridiculously marginal problems. The defensive alliance became quite extrovert and asserted the right to keep the neighbourhood (not jsut its members) safe.
A focus on tactical to strategic defence was and is enticing for national security planners who want to make do with an affordable or even small budget. Sadly, there doesn't seem to be the right technology to make it work.
The good thing about all this is of course that we Europeans are productive enough to afford a degree of military superiority over our neighbours that discourages even the attempt of an arms race against us. This strategy for preserving peace and providing security does of course only work if your power/alliance can sustain a vastly greater military might.
Asia might not be able to rely on this concept. It might instead depend on geographical barriers (Himalaya, Pacific Ocean). The defensive alliance concept could help Asia as well - but they should avoid NATO's mistake and not appear too united. That is of course one of my rather "twisted" ideas and extremely unlikely to ever gain much ground among pundits ...
Finally, one more remark: The invention of "how to build a defensive army" - be it hardware, tactic or operational art - such an invention would be worthy of a Nobel Peace Prize and easily the greatest thing of the century.