Listen carefully

America does not have "defense spending." If it did, I wouldn't be reading article like this:

The U.S. military is not prepared to meet catastrophic threats at home, and it is suffering from an "appalling gap" in forces able to respond to chemical, biological and nuclear strikes on U.S. soil, according to a congressional commission report released yesterday.

So no, America doesn't have a $500 billion "defense" budget. Defense implies actually defending something. What America has is a war budget. A killing other people in other places budget. America spends on offense -- killing people overseas. And, as the numbers in the last link show, the US spends big at killing foreigners. It has a half-trillion dollar "we think losing one war at a time is for pussies" budget. It has an Apollo-times-four "Muslims should expect to have their shit fucked up" budget. America has a Manhattan-project-times-twenty-five-sized "threats come from anywhere, so we need to be everywhere" budget.

Unsustainability doesn't just mean coal.