Unfortunately, it's a rather simplistic formulation. There's no reason to assume that some groups of terrorists couldn't fight us abroad at the same time as other groups of terrorists try to fight us here. It's not like there are just some finite number of terrorists in the world and once we get rid of all of them terrorism will disappear. Thousands of people coming across our porous borders is not helping protect us against possible future attacks. It's true that we can't completely seal the borders. However, there are steps that could be taken to greatly reduce the flow. Unfortunately, this administration seems unwilling to take those steps.