There are Shias and Sunnis living in harmony in more than 50 Muslim countries, and as diaspora communities in almost every nation. What is the difference between these countries and Iraq. Think about it. Who is providing weapons to fringe groups to kill each other? U.S.A.
Law and order, established government - especially, government established by others, so that neither is oppressed relative to the other. They "live in harmony" with each other in the US, for example. But not in Pakistan. You are certainly correct, and make an important point, in that the US foments violence and exacerbates ethnic strife in countries its corporate interests have an interest - and then blames the victims. But arming and exacerbating is not the same as inventing. The US did not invent conflict between Shia and Sunni in the Muslim world.
Clearly, you are not thinking. You are really a piece of work DH, in that you blame the split of Shia and Sunni to a country that never even existed when the split happened. And for your information, they are warring in more countries than just Iraq.
It did not invent it, much like it did not invent the conflict between Catholics and Protestants, but it does keep exacerbating the situation by bombing the population of a country into the next country or by wiping out wedding parties and villages.