On June 10, 2014, Mosul, which is Iraq's second-largest city, fell to ISIS as the Iraqi army fled. Today, Iraq's national military is in even worse shape, and may have as few as 5,000 "effective troops" under its command, according to Politico.
Despite over a decade's worth of US training and military support for the Baghdad government, along with 11 months of US-led coalition airstrikes against ISIS in Iraq and the mobilization of Iranian-backed Shiite militias to fight the jihadist group, ISIS has continued to gain ground in the country.
So it seems Iraq will have a difficult time dislodging ISIS so I think the Saudi's need to try something bold. They have a fairly powerful military and share a land border with Iraq. So a few tank armies could fight against ISIS and might send them back to Syria. They might even push into Syria as well.