"Were the Soviet Union to sink tomorrow under the waters of the ocean, the American military-industrial establishment would have to go on, substantially unchanged, until some other adversary could be invented. Anything else would be an unacceptable shock to the American economy."
https://en.m.wikiquote.org/wiki/George_F._Kennan
Bitter truth, it seems, is that without making military supplies and equipment, without using them (killing someone), without the military industrial complex the whole US economy would undergoe "a shock". Therefore a boogie man (Russia, China, Serbia, Libya, Syria, Iraq, Iran, North Korea, Vietnam...) is needed.
Also, before Facebook and along with it, there is a warmongering US media (90% of which is held by five corporations) to sell a war to US citizens.