Massive military spending creates peaceful times
Peaceful times create low military spending
Low military spending creates fear of invasion
Also the Canadians are the only ones who ever burned down DC.
Fear of invasion creates massive military spending
Massive military spending creates a need for justification, and ability to invade
The need for justification, and ability to invade create wars abroad
Wars abroad create enemies and fear of invasion
I can think of a lot of previous hegemons who got complacent and lost everything.
But alas, here we are.
I see no reason to believe that democracies with women's suffrage and public education are immune to military competition. I certainly wouldn't want to risk massive upheaval of the world order which jeopardizes the existence of all those things to find out.