Apologies in advance, [RAMBLE MODE]
Fair point but there is a law of deminishing returns. If your emaciation is progressing at the same rate as the opposition; great. Arithmetically, though, the percentage difference in capability is reduced.
We were fab once. Twatted the Spanish, courtesy Frannie D. Lizzies dad Henry had the idea of ousting the bish and creating the Andrew, relating to your last point of militarism begetting riches. Saw the French off through alliances with the Dutch, Prussians, and even the Irish. The Germans? Well, some success in Mesopotamia and Syria notwithstanding, a bit of a draw first go, Jutland for one example, Ypres and the Somme bore witness to the futility of massed units. A test drive in Spain for Franco and wahey, modern mechanized warfare is born. Were it not for Turing we might not have had the supplies our generous friends kept sending, but even their deep pockets were months, even weeks from being empty by the end in the Pacific.
There is an implosion point; say, crush depth. You're big enough to have a go but not big enough to win. Unless, of course, you're a bit player in some other Nation's political ambitions.
Some might say this influenced Imperial Japanese thinking when they went from some local invasions in Siberia, China and the islands, to poking the enemy with the big stick and oil, with its (the Japanese) arguably superior aircraft and tactics. Could have gone their way too but for some individual bravery from search aircraft crews and more codebreaking that did for the Kaga, Akagi, Soryu and Hiryu, and put them on the back foot for the next few years, ending with Little Boy and Fat Man paying a visit to end all visits
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Treasury people and like minded economists argue that we are a military power because we are a rich (relatively, yes, still) and influential Nation. It never occurs to them that we might be a rich and influential Nation because we are a military power.
With the rise of the BRIC countries, esp India and China, should we be worried that they acquire our old assets and try to copy our tech? If we could put aside our differences in Europe (ha!) we would be economically more powerful than the USA, and so might it follow we could afford a military to suit? But even the only superpower is finding the spending of 6% of GDP (depends who you ask/believe) unsustainable when healthcare is robbing 29% of your electorates hard earned. If only those cheeseburgers weren't so damn tasty...
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In short, Lord West is as right as ever. How he must have wished for some 'Tooms on CAP in his day, and now I bet he'd take a used Nimitz class and some Hornets, instead of the gaping hole we left ourselves with.