> Who gets to make that determination?

We do. Because we can. That is how the world works. If you don't make the decision, it gets made for you in the other direction.

> Also please don't pretend any of the US military engagements since WW2 had any other purpose than serving national interest.

This is a pretty childish sentiment. Reasons for war are complex. The world is actually a complicated place, and admits multiple concurrent motivations.

> any other purpose than serving national interest

Spreading democratic and egalitarian values is in the national interest.

As much as I'm extremely reluctant to endorse the actions of the US government and military, I don't think the right answer in cases of moral uncertainty is "I guess we'll just leave things as they are and hope they turn out for the best".

> You seem to be implying that it's perfectly fine for the US to bomb a family home if a terrorist is in there, accepting civilian casualties, because they might harm the population?

Yes, just like you should throw the switch on the trolley car to kill one person instead of let it kill five.

Other people don't agree.

The US's military engagement in WW2 and prior was also primarily to serve national interest.