go off half-cocked

Etymology
From the days of flintlock and caplock firearms, where the half-cock position of the hammer was both a rudimentary safety, and the proper position for priming the pan or inserting a percussion cap. The phrase was originally rendered, "to go off at half-cock."

Verb

 * : To take a premature or ill-considered action.
 * Make sure none of your men go off half-cocked and ruin this operation.