Kraut as a colloquial term for a German goes back to 1841 in print: Mercier, in "Man-of-War," One of them hit the old Crout, the Dutchman ... hit him in the eye."

1864- C, W. Wills, "Army Life," p. 309: "Some puppy finally cried out "kraut," and another echoed it with "kraut by the barrel." [General Osterhaus] wheeled his horse and rode up to us, his face white with passion. "Vat regiment is dis?" No one answered... Yelping "sauer kraut" at a German is a poor way to gain his favor."

1918- Casey, "Cannoneers (Aug. 26): "Dead Krauts and dead Yanks all over the place."

The complete OED (with 1985 supplements) doesn't include these terms for foreigners (e. g. frog, kraut, etc.)or gives them bare mention. The policy may have changed for the latest edition, but I haven't looked at it.
One did sneak through- under frog, it is noted that the word was a disparaging term for a woman, and for a Dutchman [!].