I'm currently reading "The Battle That Stopped Rome" about the Battle of Teutoburg Forest and the author Peter S. Wells writes, "Arminius was born sometime between 18 and 16 B.C. The year is uncertain, because it depends on the interpretation of an unclear passage in Tacitus's writing about him." What was the "unclear passage" that Wells is referring to, and how does that lead to uncertainty regarding what year Arminius was born in?
In Annales II 88 Tacitus states that Arminius lived 37 years and was 12 years in power ( septem et triginta annos vitae, duodecim potentiae explevit) but doesn't say, when he was born. It's possible, that the 12 years in power start in 9 AD, when Arminius defeated Varus, so that Arminius would have died in 21 AD and hence be born in 16 BC. But it's also possible, that the 12 years start in 7 AD, when his father died and it was his turn to become prince of the Cherusci, so that he would have died in 19 AD and been born in 18 BC respectively.