When was the last time that invading somebody for their cash was actually cost effective? Early Roman Empire? Modern war is damned expensive; plus it tends to play scorched earth with various flavors of stored value (human and physical capital destroyed, fiat money's health contingent largely on who wins(and there's always the alternative of just printing what the opposition has squirreled away into worthlessness rather than trying to grab it), markets for assorted intangible assets disrupted, you are basically back to stealing big chests of gold coins, if you can find any). Plus, unlike the good old days of barbaric plunder heaps, modern weapons are so costly that you had better plunder quite efficiently indeed.