The Roman Empire in 100 AD under Trajan was Rome at peak power. The Rhine River is the traditional dividing line between Roman culture (France) and Germany, but notice the streak through northern Holland.
Roman territory fluctuated, but it's been clear for years that the Rhine was never their limit. Trajan was one of the most militarily successful, but his boy-preferring successor, adopted son Hadrian, wanted to take it easy the rest of his life, and had to be talked out of giving up the hard-won Dacia. Trajan had whipped the Parthians and (as shown in the map) had added the province of Mesopotamia, but Hadrian immediately gave it up.
Hadrian's best known now for the wall, which was actually a reimagined, better-built rebuild. Annnnd, before anyone starts up, it had nothing to do with the Scots, who didn't leave Ireland until the Romans had been gone from all of Britain for about a century.
When Diocletian ended decades of decline, he also set up a system of succession, and divided the empire into east and west for better administration. Each half was also divided, such that the outermost quarters were administrated by the designated successor of each of the emperors. The western part of the western empire had its capital in Trier.