When you allow immigration without assimilation.
Warning: This is gruesome.
It'll happen here, if we don't change the way we handle those who want to move here from other countries. It has happened here...see also the Tsarnevs and Major Hassan...and countless Mexicans in the southwest on a smaller scale.
If we don't do something about the situation soon, it'll get worse.
Notice that it took armed police to stop them. No bystanders were, of course, armed.