Dic: force someone's hand

• He blamed the press for forcing his hand.
• We didn't know what she was doing until Tom forced her hand.
• Information dug up in the investigation forced the president's hand, and he resigned much sooner than anyone expected.

1. PHRASE : V and N inflect | If you force someone's hand, you force them to act sooner than they want to, or to act in public when they would prefer to keep their actions secret.
2. to force a person to reveal plans, strategies, or secrets. (Alludes to a handful of cards in card playing.)
3. to make someone do something before they are ready.

.... Cobuild, http://idioms.thefreedictionary.com/force+hand