Tactician Card +1 Cavalry leader question

If you attack an infantry unit with a cavalry unit with attached leader, and you hold a +1 Cavalry Leader tactician card; can you wait to play that card until after the infantry decides to form square or not. I'm asking because if you have to play the card when the attack is announced and the infantry forms squares, then that card is wasted. However if you wait until after the infantry makes their decision, then you can play it if the infantry chose not to form square and you get the +1 bonus.

A Cavalry unit with an attached LEADER, will combat with 1 additional die. Play this card when the melee combat is announced.

Clear and not interpretable! And it´s a risk of the attacking player for sure, therefore he should check exact, when he uses this card.
Otherwise you can force a defender easier into a square (which is most a advantage for the attacker, if the attacker play it right) or the defender must
take a higher risk, if he refuse a square (because he could lost the "last and only usefull" card).

Different use e.g. is the "Break the square" card
www.commandsandcolors.net/napoleonics/th...reak-the-square.html

I had a lot of situation, where i wanted to to force a defender anyhow into square, so this card would help.
E.g. the defender can only go with one/last unit into square, but the attacker has 2 CAV units left to battle.
So i would try to force the unit into square, which is not important for my planned attack, and then attack the other unit with full power.
If you understand what i wanted to say with my bad english

Such a use of this card is then not wasted, it makes the different of the high and low level skill players