And the peak of the Ottomans was passed by the 1730s. Belgrade, get on a boat from Constantinople and sail up the Danube. Alger, get on a ship from Constantinople and sail right there. Mecca, get on a horse in southern Anatolia and ride along the Mediterranean then Red Sea Coasts until you arrive at Jeddah. The Ottomans' realm was very easy to rule from Anatolia, there were no logistical feats equivalent to those needed for ruling north of the Caucasus or campaigning east of the Hindu-Kush.**During their peak the Ottomans fought the Portuguese in the Indian Ocean, Italians, Spanish, Austrians, Persians, etc. A power which controls Belgrade, Alger and Meca is definitely global power.
Which brings up probably the biggest point working against Iranian empires. The natural borders of a power based in the Iranian plateau are pretty bad* as far as forming big blobby empires goes. Deserts to the north and (depending on how you define the natural borders) west, large mountains to the east and north west, the Persian gulf in the south which is easily subject to blockade...
*Unless it's just the right time (as it was for Cyrus) and the Levant and North west of India are lacking powerful rivals, in which case you rule the centre of Eurasia and are free to expand as you see fit.
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**which is not to say the Ottoman Empire wasn't expansive or impressive, just that the location of its core region facilitated rather than limited its growth.
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