You can't close the borders just like that,.we are not in the Stone Age where communities live in enclaves and are also self sustaining. We now live in a global village where each country interacts and corporate with each other. The IS also depends on foreign labourers to run their industries and other sectors. Donald Trump's target for the proposed closing of the borders is not because of legal immigrants but those who come in through the back door. He is concerned that these illegal immigration contributes to the increase in drug trafficking and crime. You cannot also expel all the immigrants because some of them have the necessary documents to remain in the US. But I support the extradition of illegal immigrants because some of them pose a threat to national security.