A Roman officer
Arminius was the son of a Cheruscan chieftain, sent to Rome as a hostage or for education, granted Roman citizenship and equestrian rank, and given command of Cheruscan auxiliaries in Roman service. He knew the Roman army from inside it.
That is the fact that explains the Teutoburg Forest. The ambush of 9 CE worked because its author understood Roman march discipline, baggage arrangements, route selection and the assumptions a governor would make about a friendly auxiliary commander's intelligence report.
The destruction of Varus's army
In 9 CE Arminius reported a rising in a distant district to the governor Publius Quinctilius Varus and offered to guide the army there. Varus moved three legions with baggage and dependants off the prepared routes into broken, wooded and waterlogged country.
The attack came in stages over several days on ground where the Romans could not form a line or fortify properly. Legions XVII, XVIII and XIX were destroyed and their numbers were never reused — an administrative silence that is itself evidence.
Afterwards
Germanicus campaigned east of the Rhine between 14 and 16 CE, recovered two of the three lost eagles, visited the battlefield and buried the remains, and fought Arminius at Idistaviso. Rome nonetheless abandoned the project of a province beyond the Rhine.
Arminius's later career is known only in outline: conflict with his father-in-law Segestes, who had warned Varus against him and who surrendered Arminius's pregnant wife Thusnelda to the Romans; war with Maroboduus of the Marcomanni; and death at the hands of his own kinsmen around 21 CE, on Tacitus's account because he was suspected of aiming at kingship.
Tacitus gives him an obituary calling him the liberator of Germany, and notes that he was still sung of among the tribes but was unknown to Greek historians and inadequately treated by Roman ones.
Reception
From the sixteenth century Arminius was adopted as a German national symbol under the name Hermann, and a colossal monument, the Hermannsdenkmal, was erected in the nineteenth century in the context of German unification. The figure was subsequently used by German nationalist and later Nazi propaganda.
That history is a fact about modern Germany, not about 9 CE, and this platform describes it as reception. Nothing in the ancient evidence supports reading Arminius as a national founder: he led a coalition of tribes for a limited period and was killed by his own people.