> It pays American soldier salaries

It pays soldiers to do "useless" work. These same soldiers could have been working as construction workers, teachers, factory workers, etc doing something productive. Instead, they've spent their lives killing and getting PTSD.

> it buys products from American defense contractors that provide hundreds of thousands of jobs

Once again, non-productive goods. Instead of building missiles and bullets, the resources could have been used to build new trains for example.

> it funds R&D that trickles down to the rest of society.

Don't need war for R&D. R&D money was going to be spent regardless.

It's about the opportunity costs. Money spent well vs money spent poorly.