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One issue is that this card does literally nothing in games without attacks, or in games with only weak attacks where it is better to ignore the attack. A lot of random setups won’t have any attack cards at all, so you should account for it.
The other issue is that if this counter is effective enough, then it will just make it so that people will never choose to buy an attack card when this is available. In which case no one would buy Standing Army either. So it just makes multiple cards not even an option in the setup.