Same way the Germans attacked France...through Belgium.
Through Basra?
The Kingdom better be very careful here. Geography points to battleground Iraq and the Kingdom, even with the Gulf states, and that is prblematical would be sorely outmanned. Their far superior air assets are a plus, but wars are won on the ground and if the battleground is Mesopotamia the Kingdom is in real trouble.
I didn’t see that they expected it to be a land war. It could be an air/missile war, with SA intending only to beat Iran down enough that they’re no longer much of a threat.
One country can do a D-Day like landing to the other.
The Iranians already have proxy forces along the border. There are the Houthi (Shia) rebels in Yemen, and thousands of Iranian backed Shiite militia currently in Iraq. The army of Iran is 3 to 4 times the size of Saudi Arabia’s.
Iran is in Syria and Lebanon. Hezbollah is their proxy army.
https://www.nytimes.com/2017/08/29/world/middleeast/iran-missiles-lebanon-israel-.html
It’s more likely Iran would invade the Saudi’s. Southern Iraq is majority Shia, they might cheer Iran on.
And, the Shia minority in Saudi Arabia mostly live along the Persian Gulf, in and amongst the oil wells by the way. If the Shia in Saudi Arabia start raising hell they could well hamper oil production, the Saudi’s only real source of income.