The White opening move "a3’ is called Anderssen’s Opening. The move is harmless and not as bad as its reputation. White has basically wasted their first move, and is now playing a Black opening as White, which is not really that bad. Black should avoid thinking that it’s now a crush for Black. For example, after “1.a3?! e5, 2.c4” it has become a Reverse Sicilian (Kan Sicilian).
Diagram: Anderssen’s Opening (1.a3!?)
Black might also consider playing “1..d5” against this opening, with the positional idea that it stops the “a3” move being useful to White after a later Bc4 move. But there are, of course, plenty of other opening lines and setups for White where the move “a3” has some value.
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