The idea of Manifest Destiny is about

Manifest Destiny, a phrase coined in 1845, is the idea that the United States is destined—by God, its advocates believed—to expand its dominion and spread democracy and capitalism across the entire North American continent.

Explanation:

There are three basic themes to manifest destiny: The special virtues of the American people and their institutions. The mission of the United States to redeem and remake the west in the image of the agrarian East. An irresistible destiny to accomplish this essential duty.

to act quickly in times of danger to the united states

because you have to act quickly if you want to live/survive.