I think the outlanders become a problem when just laws are not upheld, there is a welfare state siphoning money from the inborn folk, and armed defense is restricted.
Britain's got it bad, but the US has the same issues to a lesser extent.
Discussion
Thats an intentional government problem, not an migrant problem
I agree - first best solution is to become peaceful (not helpless) and trade with others.
Second best solution is to exclude outlanders. Open border + welfare state = systematic looting of inborn folk.
I think the real issue is that the military-industrial complex has created the very conditions that lead to massive influxes of "outlanders" in the first place.
Don't want a bunch of people coming in and taking jobs away from those born and raised in any country? Maybe don't destabilize and destroy their homes and create reason for them to emigrate in the first place.
No matter how lenient or strict Customs are, the issue of immigration would be quite minuscule if the State just stopped setting up other states for failure.
IDK about "real issue" but that's a factor too