Answer:
Its location near Celtic and Germanic tribes
Step-by-step explanation:
The Romans had big challenges ahead of them in order to expand, and after that to be able to control that territory if they expand, and the main reason for that were the Celtic and Germanic tribes around them. Both, the Celtic and the Germanic tribes, were tribes that often engaged in war, they were strong warriors, had numbers on their side, and were not people that were going to allow easily someone to control them and restrict their freedom. The only advantage the Romans had was that their military was all professional, had good training, excellent tactics, and acted well as a unit. In order to be able to cope with these tribes, the Romans attacked one tribe at a time, thus avoiding fighting against all at once, and eventually it led to their success. It has to be mentioned though that there were numerous Celtic rebellions against the Romans with lot of casualties, and also the biggest and most embarrassing defeat of the Roman army was from the Germanic tribes that slaughter 3 legions of Roman soldiers.