I'm not claiming it is, nor am I arguing that culture isn't also a factor. I'm saying that military presence shouldn't be discounted, as the parent seemed to be doing.
Realpolitik is meaningful; and everyone on the left seems to have forgotten about it (I say this as an unabashed progressive liberal).
The US’ prime advantage in the 20th century was the fact that basically the rest of the industrialized world was completely destroyed twice — and then they paid the United States to help them rebuild. It wasn’t until the 70s that the US’ reliance on oil became heavy enough to exploit strategically, but by then we had built an entire post-industrial economy a decade ahead of Europe and two decades ahead of the rest of the world. We helped Japan do the same because they were basically a puppet state and gave us a foothold in Asia to serve as a buffer with Russia.
Does that sound like cultural expansion driving the military, or the military goals driving the cultural expansion?
The list goes on and on.
Defense spending is a small parasite on an enormous private market for cultural and consumer products, of which the U.S. is and was a major exporter, respectively.
Added:
Defense agencies drive innovation because they have external constraints which force them to try to procure things which haven't yet been invented. It's similar to the way that automobile races are used to drive auto innovation. Necessity is the mother of invention, and necessity is not unique to defense, but defense necessities tend to produce extreme engineering efforts.
I think people often have that twisted. A lot of the U.S.'s expansion is cultural, not military. That is to say: people liked what they were doing, and became a fan. Think: music, movies, and to some extent, literature, and now you might add software and consumer electronics to that list.
And even when looking at Rome, can you imagine how much harder those repeated annexations would have been if the Roman culture wasn't respected?
I'd say that being capable at least of defending oneself, independently, from most individual powers and believable coalitions, creates a certain amount of stability.