hostage to fortune

noun2

1. •
   An action, promise, or remark that is considered unwise because it could be difficult to fulfil or could cause trouble later on.

   • He was very cautious with his words and gave no hostages to fortune.
2. •
   A person or thing whose fate is seen as dependent on chance or luck.

   • Once several good outfits are prepared, any one of which can attack a Japanese-held base or island and beat the life out of the garrison, all their islands will become hostages to fortune. — 1950