lord of the rings - Why didn't the Istari participate in the Angmar wars?

They probably did, but if so it's not directly recorded.

In the Istari material in Unfinished Tales we read of Gandalf (with my emphasis):

But the last-comer was named among the Elves Mithrandir, the Grey Pilgrim, for he dwelt in no place, and gathered to himself neither wealth nor followers, but ever went to and fro in the Westlands from Gondor to Angmar, and from Lindon to Lórien, befriending all folk in times of need.

So it's therefore almost certain that Gandalf at least was active in the resistance to Angmar.

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