Defense seems always the most economic way when the defender is resourceful. At the same time offense seems a better strategy for attackers who have nothing to lose.
And when your capital city has a unique geography that makes it both difficult to attack and guaranteed to be a center of commerce.
I love all the giant old stone walls in Istanbul, the place is filthy with them. The people are totally used to it too - it's trippy to see schools and homes built around centuries-old ramparts.
Regrettably, that tactic is properly near impossible to use today - the US can't defend her own borders, and the world is so interconnected that stopping all foreign trade would result in mass starvation.
The wall doesn't need to be physical. It works as obstacles to slow down movement of the enemy.
For example, Google has built up its own wall by making practical extremely large computing platform. That's a big wall for them to deter competitors. Apple focus on user experience and that is a big wall for them to fend off copycats.
If U.S. has to build a big wall. The wall has to be something that make U.S. an aspiration for those enemies of our enemies, such as rebel students in Tehran or Beijing.
The U.S. has awesome walls: the Atlantic and Pacific oceans. They saved our butt numerous times in WW2, and arguably are the reason that we're a global superpower today.
Sure, but today all it takes is a few people determined to kill themself - with all the technology that is available today, the power of a small dedicated group is vastly bigger today that it was 65 years ago, and it is only likely that it will get even bigger.
The advanced militaries of the world (British Army, USMC, IDF etc) are set up for maneuver warfare. This is what is known as third generation warfare (see William S Lind), and it has its roots in the Blitzkreig practiced by the Germans in WW2. When a third-generation military meets a second-generation military (one that thinks in terms of position-holding and massed formations) in a stand-up fight the result is always a rout (e.g. Six Day War, Gulf War 1, even Rommel virtually conquering France with a single armoured division). The Soviets knew this. Their genius strategy in WW2 was to force the Germans to fight in a second-generation style (and the Germans fell for it). This is also a major reason the Soviets were reluctant to enter Western Europe. All Western militaries thought very hard during the Cold War about how to defeat the Red Army, but the Prussians knew it all along.
Unfortunately the Taliban are a fourth-generation military (adept in psyops and propaganda, insurgency, guerilla tactics, information warfare) and the West is not well configured for that (and can't become so easily, as there are still second-generation potentials enemies around - we have to think in third-generation terms still).
Aerial bombardment of population centres works fine, collective punsihment, having minimising enemy military and civilian casualties come behind
1. Casualties you care about
2. Cost
3. Extending treatis meant to only rule under reciprocity to people who don't give a fuck about the Geneva Convention.
If you can't follow standard U.S. Small Colonial War tactics pre-WW2 (kill them until they give up, then and only then give them sweeties) you can use the Strategic Hamlets doctrine that worked fine in Vietnam, and in Afghanistan for the Soviets after they7 stopped trying to run the country and would settle for not having any one unsympathetic to them running it.
200 years ago it would not even occur to Joe Afghan that he could advance his strategic goals by communicating directly with the civilian population of his enemy. Now the Taliban has it own channel on Youtube. That's why I say they're 4th-generation; this is high level information warfare and it has very successfully fulfilled the strategic goal of preventing the West from bringing its full firepower to bear.
Reagan bankrupted the USSR by forcing them to compete with Star Wars. Will the Taliban bankrupt us by forcing us to spend all our money on UAVs? Maybe not, but they can still make making war on them prohibitively expensive.
Joe Afghan doesn't have strategic goals, he wants to live his life in relative peace. Joe Afghan Warlord really is in much the same position as they have been since the arrival of the firearm; they have warbands based on a fluid and changing mixture of religious, linguistic and ancestral ties.
The Taliban does not exist in the way that the US government exists. There is no continuing organisation with a command infrastructure, the US and its allies are fighting the Pushtun culture.
Your first paragraph above, I conceded before you wrote it
"As both the Soviets and the Americans show this probably isn't possible for demotic states. Imperial commitments aren't popular."
Will the Taliban bankrupt us by forcing us to spend all our money on UAVs? Maybe not, but they can still make making war on them prohibitively expensive.
I have studied military history for less than sixty hours total. I feel confident in saying that the Afghan war is unwinnable as it is, and will continue to be waged, for the same reasons Vietnam was unwinnable. The US will never win a war against guerillas, ever, because the tried-and-true strategies that work are illegal under US law, and far more importantly, repulsive to the US populace. Oh, and because the US's only existential war was with itself. That a country with an ocean to the sides and mostly deserts to the top and bottom has the world's biggest military is ridiculous.
Seriously though. How to Win the War in Iraq
Any time your convoys are so much as fired upon bomb the nearest population centre into a smoking crater. This'll even work if you evacuate everyone in said population centre before you destroy it. This way, you don't have to do intelligence work to figure out who did whatever it was that pissed you off. Everybody nearby has an incentive to dissuade, lethally if necessary, the local hotheads.
Collective punishment, it works.
The reason the Prussians knew how to defend against the Soviets was that they where the closest thing to an intellectual heir to Sparta - most of what they did was focused on making their population ready for war.
Every empire will eventually end. The fact that Byzantium held out as long as they did is a testament to their skill. They never did try to recruit the Ottomans and mostly played with the bordering nations between them and the Ottomans. Once the Ottomans expanded and took over those buffer states, their indirect strategy was less successful. Had support from Western Europe actually materialized, things may have turned out a bit differently. Oh well, that's the way history goes.
The Byzantine empire had its ups and downs like any empire. It lasted longer than Roman Empire (per conventional definition of the term).
It is a shame that the Western education system downplays the significance and influence of the Byzantine empire on the world. The period known as the Dark Age, was only "dark" in the Western Europe. During the period, Byzantine Empire was a refuge for civilization.
Regarding IV. I think to maintain an empire/vast property that you inherited from ancestors is not that easy. You can see a lot of successors who flush the estate down to the drain just in one century. For Byzantine Emperors who were capable of maintaining those assets for almost 10 centuries despite competitions from Muslims and Crusaders is a pretty good achievement.
Well, the romans have it all written down as well. In the writings of their most prominent thinkers. To state otherwise, is just plainly wrong. You can learn so much from them, you just have to dive in and read a couple dozen books first, instead of all the wisdom being lazily presented to you in just a single manual and a single book. Far easier to learn, check. Far worthier to know about? I have my doubts.
