Right, but "closing borders" is dramatically easier when you're an island. You have formal ports of entry, your infrastructure is set up such that you have a cargo ship unloading at a port and then someone else drives the cargo to its destination. Consider, for example, goods produced in Poland and delivered to central Germany--there's no neat handoff between different delivery personnel at the border, unlike at a shipping port. Further, consider how many millions of people live on one side of a European, Asian, or American border and work on another side--how many Australians commute outside of Australia in their daily commutes or vice versa?