I was reading where Bobby Fischer used to play the KIA against the French, but haven't seen where anyone else appears to have used it often. Is this a viable way to confront the French, or he was successful just because he was Fischer?

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I think Glek played it too.

Both. It worked on a world class level because he was Fischer, but look at the database GMs are still playing it against the ever annoying french.

The KIA still gets lots of play vs the French, even at the 2600 level.  Ditto against the CK, and the e6 Sicilians.

It's really only a weak, system-y attempt by white when he tries to force it in against every response by black.  Used selectively, it's fully mainstream and viable.

It's rather better against the French than against the Caro Kann.