@ egslim: nuclear weapons have not taken war off the table.
Not war in general, but war between Great Powers has become prohibitively expensive, in a way that is recognized by everyone. Unlike WWI, when few recognized the great cost that would occur beforehand. Or WWII, when Hitler thought Blitzkrieg offered a cheap solution.
Of course war is still on the table, but only in the form of smaller-scale conflicts.
Your pretense that the relationship between the two has changed is empty
No, it is a direct consequence of changed economic realities.
Over the past several hundred years the effective taxrates in all nations have increased tremendously. It used to be that a government could wage wars until bankrupcy loomed, make peace, and be ready for the next round a few years later. That's because the government's effect on the economy was so small (low taxation, low expenditure), that the national economy would be barely affected by the government's bankrupcy.
The increase in taxation, due to inventions like the income tax and a more effective bureaucracy, allowed governments to raise more money and hence fight wars more effectively. The downside was that wars could now financially exhaust the entire national economy, instead of only the government's treasury.
So the cost of war as a precentage of the economy has blown up.
But the ability to recover said cost from loot, tribute, favorable economic agreements, etc, has not. If anything, looting has become less efficient than it used to be, because proportionally less wealth is nowadays tied up in easily lootable goods like gold and jewelry. Tribute no longer works, instead the defeated country is more likely to need foreign financial
support.
the richest nations, who have the most advanced means of industrial age slaughter to hand are also the ones who have the most to lose
Strawman. As long as they only invade weaker nations without strong allies they have nothing to fear from a counterinvasion, and they can pretty much limit the financial cost beforehand by withdrawing when the invasionbudget is spent - at the cost of a prestigehit.
This hard power: it's bloody expensive when chosen, no matter your century.
False. Ask the Vikings, hard power was quite profitable to them. As it was to the Ottoman Empire, who based their economy on the profitable use of hard power: The treasury depended on loot and tribute, its bureacracy on captured slaves.
Of course soft power is more often the tool used, it's a cost benefit winner in most cases. Having recourse to hard power, be it latent or an over threat, also opens options when applying suasion to achieve an end.
Having hard power is nice, but also expensive - even if it's not even used. One has to weigh the cost of maintaining it against the advantages it gives.
And there is always the danger that having hard power by itself entices politicians to use it, even when it's not profitable.