Saudi Arabia (Sunni) has launched air strikes on Yemeni (Shia) rebels and is reported to have 150,000 ground troops on its border with Yemen ready to invade.
Iran (Shia) has said that it "will make all efforts to control [the] crisis in Yemen". That is a diplomatic way of telling the Saudis to go home: or else.
Make no mistake about it, Iran is a very strong regional power. It can mobilise about one million well trained armed forces. It makes the vast majority of its own weapons so does not rely on imports, unlike most ME states. It has ballistic missiles capable of carrying warheads to Israel, which also put Riyadh, Jeddah, Dammam, Qatar, the Emirates, Kuwait and other members of the Arab League in range. It has its own oil and refineries. It also has troops currently in Iraq and the Saudi northern border is not very well protected.
If this really kicks off we could all find ourselves in big trouble.