Rare earth metals for one. Human capital for 2 - there are simply more smart humans outside the US than inside (simply because there are more humans outside the US than inside it.)
Inputs found outside the US sort of misses the point though - we dont live in a world that is getting less connected. Instability elsewhere will find it's way here in one form or another, hence the need for someone to underwrite security. For the moment (and for the last 90 years or so) that someone has been mostly the US. It wont always be - eventually it will be another country, or coalition, but someone will underwrite global security, or the globe will be far less secure than it is today, with the attendant costs.
Inputs found outside the US sort of misses the point though - we dont live in a world that is getting less connected. Instability elsewhere will find it's way here in one form or another, hence the need for someone to underwrite security. For the moment (and for the last 90 years or so) that someone has been mostly the US. It wont always be - eventually it will be another country, or coalition, but someone will underwrite global security, or the globe will be far less secure than it is today, with the attendant costs.