Observations from the Yr 3 maths challenge group:
We found that if you leave 3 counters after your turn then you will always win. [Because] if the other person takes 1, you take 2 and if the other person takes 2, you take 1. Both times you win.
The person who starts and takes 1 [on the first go] can always win. On your next turn take 2 if the other took 1, take 1 if the other took 2 to make their turn start with 3 counters [and you will win].
If you're first and you know the trick and don't make a mistake you can always win.
If you're second then it depends. If they take 1 and know the trick, they win. If they take 2, then you take 2 and you will win.
If you have 8 counters take 2 if you start and you will win.
9 counters:
You don't want there to be 6 counters before your turn.
If there are 9 counters and they know the trick you cannot win [because they can always make it 6 counters]