Whenever I see current affairs, I always think "how is it possible that Saudi Arabia is an ally of the US when their values so dramatically conflict with our own?" I know the answer is for geo-political and economic reasons, but I never understood why in the power struggle between Saudi Arabia and Iran for dominance in the Middle East, the US ended up allying with Saudi Arabia and the Russians ended up allying with Iran. Why was it that way and not the other way around?
The USA was strongly allied with both of them until the Iranian Revolution. After the hostage crisis and US support of Saddam in a devastating Iran-Iraq War, the relationship was severely compromised and has only recently been on the mend.
The seeds of this unraveling were sown when the US supported a coup against Mossadegh who was elected, but seemed likely to be USSR friendly. The reimposition of the Shah, who was seen as an American puppet, fueled some of the initial tension which blew up the relationship.
US didn't ally with Saudi Arabia, because it was Sunni; even that definition of the religious ideology of Saudi Arabia is arguable.
Saudi Arabia rose as a nation after WWI and the Ibn Saud's conquest of the region that followed the partition of former Ottoman states (which the precursor to Saudi Arabia, the Kingdom of the Najd, was never really a part of). In the interwar years, Britain solidified its hold in the region for the sake of oil, which grew in importance, due to the British navy's conversion from coal to oil as a source of fuel. So, Iraq was cobbled together for the sake of oil. Reza Shah was put in charge of Iran so Britain could get its oil, etc.
Here's the kicker: nobody actually knew there was oil in Saudi Arabia at the time. It's why Britain, while maintaining relations with Saudi Arabia, never actually made the effort to establish an imperial presence on the country like the other countries. Of course, it turned out that the largest oil reserve in the world was in Saudi Arabia, except that this became known close to WWII. Britain wasn't really in a position to do anything about it, while the Americans saw the opportunity to create an oil interest outside the US and establish a close relation with a Middle Eastern state.
Iran, on the other hand, became vehemently anti-American after the Islamic Revolution. And while Iran did buy arms from the US (as the Iran-Contra Scandal proved), it played the international stage in a strictly anti-American position. Of course, during the Cold War, this means that you are perceived as pro-Soviet, since the American doctrine since Eisenhower has been "if you're not with us, you're against us". This only pushed Iran closer towards the Soviets (although never actually aligned in terms of Cold War politics), and as the USSR collapsed, Russia became the neighbor to serve as a friend against the US.
In short, oil and anti-imperialistic rhetoric make strange bedfellows.
I think what was left out of the op's question but is certainly relevant to the Sunni/Shi'a split is Syria (a Sunni country, but with Shiite leadership and close ties to Russia). How did the friendliness between Russia and Syria come about? Does it just have to do with their mutual ties to Iran?