Dear Adil,

Sunnis r more hostile to the US than Shias. There are multiple reasons behind this. One reason is that US has so far targeted Sunni interests (Afghanistan or Sunni dominant Iraq) more severely than Shia interests. But when the US attack Iran, which is a predominantly Shia country, Shias wil join in with Sunnis too, with greater zeal. So the then united Muslim front would become a greater threat to US. One also must not forget the Shia majority of Iraq. In case of US attack on Iran, the Shias of Iraq would also create problems for the US.