Imagine container ships waiting at major American ports for inspection and off-loading. This is going on every day all day. Nothing suspicious about a container ship.
Then some of those containers swivel and out pops missiles aimed at buildings, bridges and other targets of terrorism. Several cities attacked like this simultaneously would put the hurt to America big time. Would we close our ports to shipping? Of course not. We cannot long survive without international trade and expect to have any kind of acceptable standard of living.
We can't treat terrorism as if it is a crime, it is war. We cannot pretend that there is no such thing as Radical Islamists, hear me Holder? If we do not stop them before they come to America then we likely be victims of many more attacks to come.