Okay, if wealth is finite in the world, we have to think of wealth as some type of goo that gets transferred around the world to different countries as the decades progress. And this method is trade warfare.
Europe was the center of the world in the 18th and 19th century. All the wealth, guns, warships, etc etc. They were untouchable.
America didn't have a significant military ability until WW1 or WW2. They were industrialists that protected their own industries and at the same time exported heavily to Europe/UK, accumulating foreign capital (gold and cash).
Europe destroys itself in WW1, all the victors become heavily indebted to the USA. WW2 occurs and all of the major European powers become puppets of the USA.
Enter pax Americana. Europe reclaims some of it's wealth through exports to the USA. Same thing with Japan. This is okay because both are in reality occupied territories of the USA.
At this point (1970s/1980s) the goo is spread between the USA/Japan/Europe/UK/Australia.
Richard Nixon teams up with China as a quasi-alliance against the USSR in the early 1970s, and as part of that agreement is increased trade. China senses this opportunity and starts producing low cost goods, accumulating US currency and building up their industry.
All of the industry that formerly was in the flyover states of the USA is now in China. Those exports markets that used to belong to American companies have now been occupied by China.
Given the above context, since no wealth is being generated in the countryside or small towns anymore. What remains of the wealth in the West is centered in the cities. And since there is no industry in most American cities, only financial markets and services which is all funded by debt.
We have to look at our major ports and major cities as sink holes where our wealth is drained to foreign countries. Frankly this is more important than having a standing military.