IDIOMS

no plan survives contact with the enemy

(idiomatic) It is a reality of conflict that after a plan against an enemy is begun, unexpected elements will always occur as a result of opposition that will require improvisation in response that will inevitably deviate from the original plan.

Example: (Can we date this quote?) Brigadier J Nazareth, Creative Thinking in Warfare:
  As Moltke the Elder said, "no plan survives contact with the enemy