This quote is the core of the "Menace" speeches. He wasn't afraid of the bomb exploding by accident; he was afraid that politicians would treat the bomb like just another cannon. He feared they lacked the imagination to understand that with nuclear weapons, there are no longer "victors" and "vanquished"—only survivors and the dead.
Albert Einstein "Peace in the Atomic Era" Transcript - Speeches-USA albert einstein the menace of mass destruction full speech