Any Western country is now potentially a target, whether they have been to war in the middle-east or not. However, a big factor in this is the wars in which we have taken part, creating broken nations in which Isis and Al Qaeda are able to flourish (see Libya and Iraq). We are also too friendly with the Saudi leadership, who share an ideology (Wahabbism) which is the basis of the beliefs of Al Qaeda and Isis... and who are happy for these groups to flourish when they fight against rival nations, such as Syria.
It might help to make the world safer if we stop over-throwing regimes and creating chaotic power vacuums and also, stop arming barbaric countries, in particular Saudi Arabia. In the end though, America is going to continue with all of this anyway and the rest of the world will suffer as a result. But we at least could/should not contribute to it.
And for years Iran was the at the heart of the Axis of evil yet they and the Saudi's are sworn enemies with both wanting to be the dominant force in Middle East.
Yet, it seems the Shia muslims are now less of a threat than the Sunni's