I am a big fan of the Monroe Doctrine circa 1823. Paraphrasing, it says, if a European Nation attempted to intervene and exert influence and power in our hemisphere (meaning the Americas), we would not allow it.
Two things to consider about this:
1. When this came out, European nations were still mucking around in Central and South America significantly - including in Mexico on our border (no surprise - this was Napolean). Europe contained the only nations with any power that could have threatened us (in any meaningful way) at that time.
2. In a 21st century world, when everyone is less than a 20-hour plane ride away, the hemisphere component of this is no longer relevant. Every place on earth is in our hemisphere. A terrorist sitting in Tehran can pick up his nuclear suitcase and be on our shores faster than my commute when the Mass Pike has an accident on it.
Given this, I am mentally prepared to take on the burdens of preventing another 9/11 on our shores or any of the other increasingly viable threats being hurled by saber-rattling Iranian mullahs and their leader -- even given what the consequences of that may be.
This seems to me to represent another angle on the "means" to hurt us. I am not very well convinced they lack desire.