And none of these would have come into Afghanistan if we weren't in Iraq?
You remember Afghanistan, don't you? Home base of al Qaeda?
These people were streaming into Iraq first because the Syrian and other pipelines into Iraq were more convenient than Afghanistan was (Afghanistan's pipeline is mostly Pakistan.)
In other words, "N + X" would be a valid equation if we had never gone into Iraq at all.
It's amazing how many people are thunderstruck at the idea that more battles occur after a war starts than beforehand.