The writings of john l. o'sullivan showed the power of to shape public policy.

The press -- mass media

John O'Sullivan coined the phrase "Manifest Destiny" to describe what he saw as the God-appointed duty of the United States to expand across the North American continent and to lead the world by the example of its values.  O'Sullivan was editor of the Democratic Review and of the New York Morning News when editorials in both publications (in July 1845) put forth arguments for annexing Texas and the Oregon Territory.  US public policy in the years that followed did essentially pursue the sort of territorial goals O'Sullivan had described, even if not always with his same spiritual ideology in mind.