Secondly, it would mean overstretching my forces. I wouldn't have enough reserves for E. Africa. Pasargadae's neighbour to the North is Russia and I feel better having Persia as a buffer between Catherine and myself.
Thirdly, I am still at war with Aksum, present-day Eritrea, and I want that city because of its access to the Indian Ocean. Aksum declared war on me so I shouldn't get too much of an unhappiness hit if I conquered it.
Finally my Empire needs to build more cities in the region - where today's Basra and Amman are in order to fill in the gaps between Constantinople and Cairo/Persepolis. In other words I need an extensive period of peace to, amongst other things, catch up on my paperwork in Rome.
The Empire around Arabia
935 AD: Several units have been given orders to march to the African front and luckily I have an Open Borders agreement with Arabia. My advance forces meet an Aksum Archer and Worker.
940 AD: Combat ensues and my Knight easily overwhelms the enemy....
....more forces are revealed.945 AD: My Longswordsman crushes a second Aksum Archer but their Knight severely mauls mine - we retreat but not to safety....
....because their Knight attacks again destroying my guy and recapturing their Worker.
950 AD: Follow-up forces have arrived and after a bombardment the Aksum Knight is destroyed and the Worker captured again.
The front is now firmly in friendly hands.
The Middle East in 950 AD. I have 5 Workers busy building roads connecting North Africa with Asia Minor. The turn ends with Thailand declaring war on India. Note that the remnants of the Persian Empire are not only N. of Persepolis but also to the East.