Of course, the difference between international and domestic terrorism is that domestic terrorism occurs primarily in the United States.
Under federal law
, the term "terrorism" refers to any violent or dangerous crimes that "appear to be intended" to either (1) intimidate or coerce a civilian population, (2) influence government policy by intimidation or coercion, or (3) affect government conduct by mass destruction, assassination or kidnapping.
The first requirement is that the act constitute a violent crime under federal or state criminal law. In a situation involving a mass shooting, that requirement is easily met. Murder is the most serious, violent state crime on the books. Murder can also be a violation of federal law in certain circumstances. The killing of a federal officer is a federal offense, for example, as is any murder committed on a federal military base or other institution.
The second requirement is where it gets difficult. The crime must "appear to be intended" to intimidate a group of people or the government in some way. Almost any mass murder would necessarily appear as an act intended to intimidate the people present in the area or anywhere nearby, but it wouldn't necessarily appear as an act intended to intimidate an entire group of people who can be identified as a "civilian population."
The law does not say it has to be the entire United States population, or that it needs to affect a certain number of people. Instead, the law simply states that the act must appear to be intended to intimidate or coerce a "civilian population." There is very little guidance from the courts on this definition, but this could presumably include any defined group of people -- residents of San Bernardino, or supporters of abortion rights, for example.
But we don't stop there. Even a mass shooting that does not intimidate or coerce a specific group of people may nonetheless be "terrorism" if it appears to be intended to change government policy or obstruct governmental functions.
Did the killings in Colorado Springs appear to be intended to influence government policy as it relates to funding for Planned Parenthood or abortion rights? Whether it can be legally classified as an act of terrorism hinges on this.
Notice that the definition of terrorism is not directly tied to the number of people killed, and certainly not the race, nationality or faith of the people committing the crime. Terrorism also doesn't require that we show the actual intent or motivations of the shooter. Instead, what is critically important is the appearance of what the shooter intended to do.
Given this, no matter how much we know about the background or motives of the shooters involved in San Bernardino, I don't think it's clear that the San Bernardino shooting is a case of terrorism. If we are still grappling with understanding why the tragedy occurred, then the intended effect is not apparent.
To define an act as terrorism we should be able to immediately identify and determine it as such. It's not something that we should have to ponder and investigate. Proving someone guilty of terrorism certainly takes time, but defining the crime as terrorism does not.
Some people have focused on whether one of the San Bernardino shooters was "radicalized." At first glance, this makes perfect sense. If the person is "radicalized" to the extent he sympathizes with a group like ISIS, then that person must share the group's ideology and agenda.
But such a focus is also misleading, because a person who happens to be "radicalized" can commit a crime without the intention of influencing a certain group of people or changing government policy. That's true even if the "radicalized" person has been in contact with people known to the government to be affiliated with ISIS, as the shooter in San Bernardino appears to have been.
"Terrorism" is determined by the nature of the crime, not the nature of the criminal.
While the legal definition of terrorism is a little vague, it does not involve any consideration of a person's faith, nationality or beliefs. If we are going to be able to fight terrorism with any degree of success, we need to first agree on what it is, and what it isn't.