In the United States, the desire for more national security after the 9/11 attacks has changed immigration policy in a number of ways. Let us look at two of them here.
First, there has been more attention to issues of illegal immigration. This is partly due to economic and cultural reasons, but it is also connected to 9/11. Illegal immigration has gotten more attention and laws have been enforced more strongly because of the fear that some of the people entering the country illegally could be terrorists. Along with this fear has come the transfer of immigration enforcement from the Department of Justice to the Department of Homeland Security. This means that security concerns are driving much of the policy on immigration law enforcement.
Second, it has become much harder to gain legal entry into the United States, particularly for people from Muslim countries. There is a fear that people from such countries will use such things as the asylum program to infiltrate into the US for terrorist purposes. This fear has made it harder for people to come to the United States.