why an ISIS-Saudi war is not only probable but inevitable

The forces that were unleashed in the Iraq War have not yet been fully realised. This is especially true regarding ISIS and the threat it brings to Saudi Arabia.

Whether the conflicts in Iraq and Syria end or not is irrelevant - either way, the international community will still be faced with an ISIS-Saudi war within the kingdom.

The two options of an ISIS spillover into Saudi Arabia are a 'defeat-spillover' or a 'success-spillover.' A 'success spillover' would mean that ISIS maintain a permanent base in Syria and Iraq and from there are able to launch attacks on Saudi Arabia and destabilize it. In the case of a 'defeat-spillover,' Saudi Arabia would lose the large Syrian-Iraqi proxy war to Iran. This would cause Saudi Arabia to destabilize in favour of ISIS.

Interestingly, Hillary Clinton's foreign policy would have resulted in a 'success spillover,' while Donald Trump's policies are likely to result in a 'defeat spillover'.

Donald Trump is planning on reversing ISIS' gains permanently in Syria and Iraq to such an extent that he is prepared to work with Russia and the Syrian Government, an Iranian ally, to do this. He is also prepared to work towards military, political and economic solutions in Iraq, which might succeed in giving the country stability for the first time since 2003.

Should Trump be successful, the ISIS 'defeat-spillover' would cause Saudis to blame their government, pledge allegiance to ISIS in overwhelming numbers to rid Saudi Arabia of its twin threat: Iran and America.

This war is only inevitable because the forces of the Middle-East have not yet adjusted to the seismic shift caused by the Iraq War. By removing Saddam Hussein, a secular Sunni, and replacing him with a Shi'ite dominated government, Sunni dominance under secular Saddam Hussein was replaced by a struggling Salafi Islam as Saudi Arabia tried to fill a Sunni void in Iraq, a void left by the overthrow of Saddam Hussein. This 'filling of the void' in Sunni Iraq gave us ISIS.

Should Iraq and Syria stabilize under regimes friendly to Iran, Saudi Arabia would need to have an internal transformation in order to resist the threat of an Iran-dominated Middle-East. ISIS is of course a natural benefactor, as Saudis will choose ISIS to force their government to act more forcefully against the threat of Iran within their country.

ISIS also gives Saudi Arabia the excuse to rid the Arabian Peninsula of all Shi'ites, likely causing large Gulf Shi'ite emigration to Iraq. This would mean that Iran would have no chance of using the Shi'ite minority in Saudi Arabia for their own gain.

Regarding America, both ISIS and Al-Qaeda provide avenues for Saudis to resist American influence within the kingdom. This would send a clear message to America: Saudis no longer trust Americans because of the Iraq War.

The Iraq War has had implications far beyond the immediate removal of Saddam Hussein. Even should Iraq stabilize, we will still see the consequences of this disastrous foreign policy played out elsewhere.