1) Civilians die (people without a choice)
2) Resources are depleted
3) Funding is spent that could grossly improve lives of people
4) Environment degradation
The USA spends, at least, more money on its military than the next dozen countries combined (ref:
https://en.wikipedia.org/wiki/List_of_countries_by_military_expenditures).
e.g. The USA could cut its military spending in half and still spend more than #2.
However its not that easy. A lot of jobs in the USA are based on that spending. How to curtail that spending without costing jobs in the aerospace/defense industry could be a challenge.