Nepos' Themistocles: ut ingratis omnes ad depugnandum cogerentur?

I am reading the biography of Themistocles by Cornelius Nepos. He recounts the story of how Themistocles used a deceit to bring about the naval engagement that went down in history as the famous Battle of Salamis, when his compatriots, seeing Athens burn, became restless and wanted to leave the ships to defend their cities.

Themistocles sends an envoy to “the king,” which in this context always seems to be the Persian king Xerxes, with orders to tell him (correctly) that the Greek are about to disperse and that he should attack quickly now when he can beat them all in one place.

Nepos then explains the idea behind this ruse:

Hoc eo valebat, ut ingratis ad depugnandum omnes cogerentur.
This aimed at driving everybody (???) to battle.

I cannot make sense of ingratis. It seems obvious to me that it is supposed to mean something like “against their will, unwillingly.” But why the dative or ablative?