Even by its official definition, the US "border" stretches 100 miles into the interior of the country — encompassing as much as two thirds of the entire US population. But obviously, even going 100 miles inland doesn't cover every point where people can enter the US. After all, international airports exist all over the country.
In all, according to a Pew Charitable Trusts report, there are 314 "ports of entry" in the US — places where US Customs and Border Protection officials can formally inspect documents and approve people to enter the country.
Security at ports of entry requires a totally different process from "border security." For one thing, while agents don't want to let anyone through at a port of entry without papers, they don't want to force people who do have papers to wait in a two-hours-long line either. But when politicians talk about "securing the border," they're almost always talking about the US/Mexico border — and they might be using a zero-tolerance definition of "security" that has no relationship to reality.
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