Let's just turn to our national ID system...oh wait...

> But a lot less could go wrong than just letting anyone into the country en mass with no practical way to remove them.

Are you suggesting the only two options are extraordinary rendition to concentration camps in El Salvador with no recourse or instant squatters rights for all foreigners?

> You effectively don't have a country at all under that regime.

Let me offer an existence proof: gestures all around to where we're currently standing

We do have birth certificates and in most states it is actually very hard to get state ID without at least residency so yes asking for ID is usually enough.