Imagine the U.S: was only half as rich. Would this mean the %GDP budget should be 4%, not 2%?

Why is the GDP a good base for determining a budget at all? To me it looks rather like a variable that tells you something about affordability than about necessity.


You could for example look at Purchasing Power Parities, military spending and inefficiency levers (strategic logistics, degree of bureaucratic inefficiency) in one bloc and compare this with your bloc (say, PRC + NK vs. Taiwan+U.S.).
Then you have an idea about the relative power situation and can continue with thoughts about wargames and such.