Gaius Julius Caesar was, in 692 AUC (62 BC), a rising star in the Roman Republic. He had successfully leveraged his popularity with the Roman people to further the enfranchisement of the other cities of Italia, with the Lex Julia. As his term as Praetor drew to a close, he was able to leverage his new popularity with the rest of the peninsula to advance his career further.
So it was that, in 694 AUC (60 BC), he served a term as Consul, during which time, he spent much of his energies planning and preparing for a military campaign to crown his career. He was not stifled for lack of options. To the north, in Gallia, the Helvetian tribes were pushing up against tribes allied to the Republic. Meanwhile, in the east, the Dacians, under the king Burebista, were launching a series of campaigns against the various independent tribes in Illyricum. Burebista also supported various Greek cities that were discontent with Roman rule, always flirting just on the edge of acceptable conduct.
Burebista had worked hard during his time as king of the Getae tribe, pre-eminent among the Dacian peoples. He was responsible for the expansion of their territory up to the Hypanis (Bug) River in the north, down to Dionysopolis (Balchik, Bulgaria) in the south, and west to Hercynia (Moravia). He reformed the Dacian military and refined the fortification of many locations, in the Murus Dacicus style, a combination of Dacian and Greco-Roman engineering.
Caesar was ultimately destined to campaign against Dacia, and soon, treaties were made with various tribes along their borders, including the Scordisci, Boii, and Taurisci confederations, all of which were feeling the weight of Burebista's armies. Caesar then was assigned, at the end of his term, to the province of Illyricum, and made the final preparations for his campaign.
He issued several proclamations demanding that the Dacians leave the allies of the people of Rome alone, while mustering his forces, gathering up many auxiliary troops and an additional legion, recruited from volunteers across the Italian municipalities. He was also able to procure the support of the Thracian king, Spartacus, a long-standing Roman ally.
As Burebista continued to defy Caesar's demands, Caesar began to march off to war, determined to conquer Dacia. Against him stood Burebista's armies, numbering nearly 200,000, as well as their allies, the Bastarnae tribes to the north. Caesar had, under his command, three legions, the Legio V Illyrica, Legio VIII Victrix, and Legio X Felix, numbering roughly 24,000, as well as an equal number of auxiliary troops. His allies included Spartacus, with an army of roughly 20,000 men in total, as well the force of the Scordisci, Boii, and Taurisci tribes. As the war began, Caesar was also able to secure the allegiance of the Jazyges, a Sarmatian tribe living in the vicinity of Dacia that had dealing with the Republic.
Caesar's army quickly reached the Danuvius (Danube) River and linked up with Spartacus' Thracians. He spent the first year of the war securing fortifications up and down the river, and building ships to patrol it. It was near the end of the campaign season in 695 AUC (59 BC) that the first major battle took place, near the Greek city of Tomis, in the region known as Scythia Minor (Dobruja). The Dacian army was able to cross the river with little difficulty, and march down one of the smaller rivers toward Tomis, where they intended to then foliow the coastline and draw as much of Caesar's army away from the Danuvius as possible.
Caesar, leading the Legio VIII, though well behind the Dacians, was able to make up the difference relatively quickly, through both forced march and by transporting most of their supplies by boat, enabling their baggage train to be carried along with great speed. Though the Dacian general, Dapyx, expected to be well into Thracia before the Roman army could reach him, they were able to catch up to his forces before he even reached the Euxine Sea.
With the rivers and lagoons to their back, the Dacians hurried to build makeshift defenses, which quickly became inundated by unexpectedly heavy rains. Though the rains slowed down the Roman advance somewhat, they devastated the Dacian fortifications. Dapyx then marched his troops, numbering roughly 40,000, against Caesar's army of roughly 20,000 (8,000 Legionaries, 8,000 Auxiliaries, and 4,000 Thracians).
Caesar ordered the Thracians, on the left flank, to give way to the advancing Dacians, while holding much of his cavalry in reserve. As the Dacians poured through the gap in the line, the Roman cavalry stormed down on them, driving them against the river, where the ground was still too soggy for effective cavalry charges. However, by this point, the Thracians had reorganized their lines and had once again made contact with the Dacians, while the bulk of the Roman force was occupied with holding down the remaining Dacian army, while the forward units were destroyed.
Now, with the numbers of the opposing forces much more equal, Caesar's army began to roll back Dapyx's, shattering the Dacian force. Roughly 12,000 Dacian soldiers were able to escape the battle, a loss of almost 28,000. Meanwhile, Caesar's forces had lost almost 8,000 troops. Through this hard-fought battle, Caesar's forces were able to secure Scythia Minor relatively quickly afterward, shoring up fortifications in the area. By the spring of 696 AUC (58 BC), the region was firmly under Roman control, depriving Burebista of many of his Euxine Sea ports. Meanwhile, Caesar also enrolled two new legions, the XI Scythica and the XII Tempesta.
So, clearly, Caesar will not be conquering Gaul in this history, but Dacia in its place. I figured that, since this is Julius Caesar we're talking about here, he deserves more detail and embellishment, so I've gone into more detail about the forces involved and the battles.
A note about the legions named: they're total fabrications. At this point, most legions were still temporary units, apparently, and so the units Caesar had in our history and this were recruited during his terms. This means that the numbering and naming of the units will differ from those used in the Gallic campaigns of our history.
Burebista, also, was an interesting character. He expanded Dacia far beyond the realm he inherited, reformed the government, improved its fortresses, and dabbled in involving himself in the various Roman civil wars of our history. Caesar was planning on invading Dacia for his trouble, right before he was assassinated. Burebista faired little better, however, and died soon after. His kingdom collapsed into different successor states, which combined weren't able to field a quarter of the army that he had. Its really amazing how many grand empire builders there were around this time. A shame, almost.