No one conquered the Cherusci. Augustus ordered an invasion of central Germania and this was initially successful. The Romans reached the Elbe and all the tribes in the area were subdued. However, Arminius, a leader of the Cherusci led a coalition with included the Cherusci, Bructeri, Marsi, Sicambri, Chauci and Chatti and routed three Roman legions in the Battle of the Teutoburg Forest (9 AD)

Cherusci, Franks, Saxons, Swabians, and the Bavarians are just 5 of more than a hundred or so tribes.

Tribes around the northern borders of the Roman Empire were the Franks, Cherusci, Chatti, Hermunduri, Alemanni, Marcomanni, Quadi Iazyges, Dacians, Costoboci, Carpi, Sarmatians, and Scythians Apart from the Iazyges, Sarmatians and Scythians who were Iranian-speakers, these people were Germanic. The Goths formed a kingdom in Ukraine, northeast of the empire.

The Visigoths, Goths of the west, asked for permission to settle in the lower Danube area of the empire to escape the Huns. The emperor Valens granted this. The Ostrogoths (the Goths of the east) settled in Pannonia (eastern Austria and western Hungary) while this area was taken by the Huns. The Romans then recovered this area. The Flanks were allows by the Romans to settle in Holland south of the river Rhine and in Belgium.

The peoples who invaded the western part of the Roman Empire were the Vandals, Sueves, Alans, Burgundians and Alemanni. The Angles, Saxons, Jutes and Frisians migrated to Britain in waves.

It might be fair to call them Germans. As to where they came from, Germany was believed peopled long ago by migrants originating from the the south and southeast. A link is provided to an article that may have some clues.

The Romans had a great many enemies over the 1,200 years of history.

They were: other Latin cities, Etruscan cities, the Sabines, the Volsci, the Aequi, the Samnites, the Umbrians, king Pyrrhus of Epirus, The Carthaginians the Illyrians, the Gauls of northern Italy, The Macedonians, the Aetolian League, the Seleucid Empire, the Achaean league, king Mithridates Vi of Pontus and king Tigranes II of Armenia, King Jugurtha of Numidia, the Cimbri, the Teutones, the Lusitans, the Celtiberians, the Cantabrians, the the seves, the Harudi, the Belgae, the Gauls of France, the Persian Empire, , the Cherusci, the Bructeri, the Chatti, the Jews, the Dacians, The Marcomanni, the Quadi, the Iazyges, the Cauchi, the Hermunduri, the Lombards, the Costoboci, the Buri, the Naristi, the Cotini, the Roxolani, the Bastarnae, the Garamantes, The Alemanni, the Sarmatians, the Carpi, the Goths, the Heruli, the Scythians, the Juthungi, the Vandals, the Franks, the Blemmyes, Radagaisus, an (Ostrogoth king) the Alans, and the Burgundians.

This list does not even attempt to be exhaustive. With so many enemies, the reason for having enemies varies from circumstance to circumstance.