Sassanids!
I started off my campaign by making peace with the Romans, paying them tribute and establishing trade rights. This left Shapur's hands free to deal with the massive invasions in the east.
The first one the Persians beat back was the Hephthalite incursion - their innumerable assaults on Merv were all repulsed by troops from the entire region. I also bribed all of those large, elite rebel armies starting in Hyrcania to fight for me in the east.
But just as his generals were doing that, the Guptas invaded. The first couple of waves were defeated in the field, but casualties forced general Khusro (or was it Kavadh?) to retreat inside the walls of Herat. There he beat back one army, and with reinforcements from Arbshahr made a bold strike at that small town just north of Herat, to secure our flanks.
He did succeed, but suffered casualties to the cavalry which could not be replaced easily, and couldn't force the 400 inhabitants to stop rebelling with his 2000 men. So he got fed up and retreated south... Just as the Guptas under Megabyzos and another guy launched another attack. He was routed and fled to Arbshahr. The Guptas took Herat.
Meanwhile, Shapur's Vuzurg Framandar, Fariel of the house of Sassan, headed into the Arabian desert with a giant army. He seized al-Hirah, and then his subordinates subdued the rest of the Lakhmid lands. With trade routes unobstructed, money started flowing in from the Persian gulf trade.
Thus the eastern satraps launched a counterattack and forced the Indians out of Khorosan. However, an attempt at the Indian capital was forced to retreat with heavy casualties by the massed elephants of the Guptas. Shapur and the Indian king agreed on peace.
The next year was spent in preparations - in the west, an elite, new model army was being raised in Mesopotamia, and in the east, hordes of nomads were hired for the war with the Kidarites.
As expected, the Romans attacked. Their attacks were repulsed, and the new model army (Arteštaran Nizagan, Savaran i Grivpanvar and Savaran-i-asavira, supported by artillery) together with the starting stacks conquered all of Roamn Mesopotamia within two months. The Romans, still reeling, threw all their reserves (~3.5 full stacks in total, although somewhat dispersed) at Callinicum. The novel strategem of Surena the horseman and Mardonius Pahlevi - swooping around the slower Roman armies and defeating them in detail through night attacks - proved to be extremely effective and won the day. The fall of Antioch and Damascus kicked off the invasion of the Levant, which could not be stopped.
Meanwhile in the east, two Persian forces made a giant loop through Bukhara, another city, Campus Sakae or whatever, and then down towards Sogdia, conquering most of the Hephthalite empire. Balkh was besieged, and the Huns sought peace. The Guptas were defeated in the field several more times, yet their manpower seemed inexhaustible. So nearly the entire southern coast of Iran was called to arms, loaded on ships and sailed past the Indus into Sindh. Barbarikon, guarded by just the faction heir and his retainers, fell to the massive Aryan army. Buddhist temples demolished, locals slaughtered (note: Buddhism was later restored after severe unrest in the region) and the treasury - sacked. The Guptas were now almost powerless, and sued for peace. Knowing that a frontal attack on the Indus cities would be a stalemate, Shapur accepted.
While that was going on, one of the northern generals, whose name escapes, me purchased an entire army of steppe nomads, and secured the northern frontier. Not before dying in a glorious charge against numerically superior forces in a fatal attempt to secure the faltering center of Hvar infantry, though - a bid which ultimately succeeded.
And here we are now - the Levant, norhtern Egypt and eastern Anatolia is under Persian rule once again, so is central Asia and parts of India. There is still work to do, however - western Anatolia and much of Egypt is still under invader rule, and the Armenians are growing ever stronger.
Pictures:
The central Asian part of Ēranšahr.
The south-eastern front after a peace treaty.
Surena the horseman, the commander-in-chief of the western front.
The western front after driving out Romans.