Failure is a Choice

How is a nation able to police the world and project force around the globe unable to defend its border? By choice. We can send cruise missiles around the world to target the passenger side of a sedan in motion but can't stop people wading across a stream? 

Every illegal alien that comes into this country comes in because this country allows him to. And everyone that suffers the harms of illegal immigration, every low wage worker that can't get a job, every addict who ODs on fentanyl brought across our border, everyone who suffers from illegal immigrant crime, does so because someone decided that something else is more important. 

I wonder what that something could be?