Actually, there's an argument wether "duytsche" means of "dutch" or "german".
William of Orange certainly was of german blood, as Hesse-Nassau is nowhere near the present netherlands. But on the other hand, back in the 16th century, dutch and german was not yet as problematic as post 1940..
Anyway, Wilhelmus was quite popular as a protestant combat song in the 30 years war in Germany as well, and apparently used as a choral even later, as the protestant states and church of Germany always regarded the netherlands as something of a role model.
But "oldest anthem" is a problem, as Wilhelmus was certainly THE national song of the Netherlands, but not the official anthem of the Netherlands until 1932.
The Kimigayo of Japan exists since the 10th century, as a song of reverence to the Tenno, but also was not a formal national anthem until the 1870s, when it was rearranged for a western style orchestra by a german composer working in Meji Japan.
I suppose the british "god save" really has to be considered the oldest. Haydns Kaiserlied (present day german anthem with different text) is also quite old, but the state it stood for (Habsburg Austria-Hungary) does not exist anymore.
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