"America must not ignore the threat gathering against us. Facing clear evidence of peril, we cannot wait for the final proof, the smoking gun that could come in the form of a mushroom cloud"

This quote by George W. Bush is referring to the risk of terrorism and the requirement for America to take action versus it. He is cautioning that America must not neglect the hazard that is gathering against them which they can not await the last proof of threat before taking action. He is suggesting that the final evidence of threat might can be found in the kind of a mushroom cloud, which is a sign of a nuclear surge. This quote is a call to action for America to take the threat of terrorism seriously and to take actions to protect itself from it. Bush is stressing the need for America to be proactive in its defense against terrorism and to not wait until it is far too late.