Let's say that Crassus destroys the Parthian Army at Carrhae, and that Julius Caesar wasn't assassinated, and that he became a much-loved ruler. And Rome won Teutoburg?
Rome: Parthian Destruction from the West
The Imperial Roman Army, now swelled to about 40 Legions, or 240,000 men, was now at Palestine. Egypt had been conquered without force due to marriage and now Rome was at total war with the Parthians. The Romans had conquered Britain at Caesar's first time and now had a 30 Legion strong Army facing the Parthians, who were still reeling after the disaster at Carrhae. Imperator Caesar Divi Nervae filius Nerva Traianus Optimus Augustus Germanicus Dacicus Parthicus or Emperor Trajan, gave his Legions the order to attack, and seeing their Emperor, they swelled with great pride. They destroyed the Parthian Army and in two months were in Central Asia, near the border of Afghanistan. But as they were preparing to advance, an army of some strange people camped opposite of them and sending envoys, an army of small eyed people. Who were they?
China: Parthian Destruction from the East
The Han Chinese Army, a massive entity of some 650,000 men, came flooding across the frontier of the Parthians and defeated them at the cost of 200,000 men. They marched into Afghanistan, killing another 12,000 Parthians in the process. And when they got there, the Army, about 500,000 men after reinforcements came, saw an army, small in proportions, just about 180,000 men, encamped on the opposite of the frontier. They were not Parthians, for they were red, the infantry in heavy armor and in mails, and their banners had some strange characters in it. Two delegates, with their translators, then went to the camp of the other army to talk to them.
Imperial Rome and Han China: Negotiations
The Romans sat on the negotiating table with their translators, and then the strange group went in too. They now understood each other, due to interpreters and the Romans soon found out they were called Chinese and that they were angry. The Romans then found out that the Chinese, like them, were an Imperial Power and that they did not want a minor share in the spoils for the Romans traveled faster then the Chinese. They were told that they would need to return to modern day Iraq, give them the spoils of the provinces, and pay them 200 gold talents per year as tribute to them. The Romans naturally disagreed and the Han Chinese told them that there would be war. The Romans, shocked by this fact, made a sentry hurry back to Rome to request for aid, supplies and instructions.
The Chinese sat on the negotiating table with their translators, and then the strange group went in, too. They now understood each other, due to interpreters and the Chinese soon found out they were called Romans and that they were an Imperial Power in the west. The Chinese, angry and shocked that they were not masters of the world and that there was another nation who claimed that they could match them, although they excelled in trade with each other, provided a series of ultimatums. When the Romans immediately disagreed to their ultimatum, they declared war on the Romans. They didn't know the consequences yet.
Imperial Rome and Han China: Preparations
The Romans sent a spy to learn about them and learned that the Chinese excelled in missile weapons and cavalry due to something called stirrups and quality and quantity out of this world. The Chinese also had weapons the Romans never heard about and magnitude so great that the Roman weapons would look like old school. They had a weapon that could fire multiple missiles and the quality, range, and firepower overwhelming. The manpower was also staggering since reinforcements, including their own men, were 12 Legions and lots of Greek, German and Dacian auxiliaries. There were also Parthian deserters that put the Roman Army of the east to about 210,000 men. The Romans then asked for artillery while they also made lots of lines for Defence-in-Depth, hoping to make it a counter to inferiority in offensive equipment and manpower. Only their armor and shields were any match against the Chinese but still inferior to their artillery. The Roman Army of the East was to hold the Chinese while they made new soldiers with newer and more credible equipment. Finally, there was a contingent of fighters from Carthage, Spain and Egypt, swelling the army to 300,000 men. They were led by Quintus Victus Eastus.
The Chinese immediately told their officers to prepare for war and called on their mobilized civilians, putting their army to 550,000 men. They also learned of the equipment of the Romans and learned of their superiority, especially in ranged troops. They then called on their best strategists, who proposed to attack along the whole front, thereby overstretching the Romans. They were led by Zhao Li Yuan.