This is a Saudi-led Sunni coalition, out to stop the Iranian expansion/ influence in the region. They were quite unwilling to form a military coalition to fight the Sunni ISIL, even though they pose an immediate threat, after carving out large areas in Iraq and Syria and threatened Jordan. This military coalition of Sunnis will lead to a confrontation with the Shias of Iraq, Syria and more specifically with (Persian) Iran.
Iran has been fighting side by side with their Shia allies in Iraq and Syria who have been supported by the Lebanese Hizbollah and have been accused of arming the Houthis of Yemen. The Saudis had wanted the Americans to help oust Pres. Assad of Syria but the Americans decided that defeating ISIL was more important, even though this could mean that Assad would benefit. The Saudis are also very concerned with the nuclear talks going on with Iran and (like Israel) would like to see the talks fail and have Iran more isolated and embargoed. The Americans, under Pres. Obama, have made it clear that the defeat of ISIL is their immediate most pressing concern and that in this they are in agreement with Iran. So too, are the talks on Iran's nuclear programme, even if this annoys the Israelis and the Saudis.
It is very byzantine, this situation. Here you have long-time and close allies, the USA, Israel and Saudi Arabia at loggerheads, while at the same time the USA's concern about ISIL is causing them to find common grounds with long-time enemies, Iran and Syria.
The Saudis then, have turned to the Arab League and they have concocted this Sunni alliance. The Sunnis and the Shias have had a inimical relationship over the past 1400 years. This has led to many violent flare-ups in many places. The Saudis and their extreme Wahabist Islam, has spread far and wide with the financing and building of mosques and madrassahs/ Islamic schools. This has been a breeding ground for extremism and has led to the creation and financing of the Taliban of Afghanistan; of al Qaeda in Pakistan, Indonesia, Yemen and elsewhere; and also of ISIL in Iraq and Syria; Boko Haram of Nigeria and neighboring countries is also of major concern, as well as "jihadists" influenced in mosques in Western countries ( there is a lot of recruiting through preaching, tapes, social media also).
The Arab League led by Saudi Arabia has launched attacks on the Houthis in Yemen. This will draw in the Iranians and this could well spread to Libya, Egypt/Sinai and eventually to a new "Karbala", which will pull in others including the West. This threatens us all.