What did The writings of John L. O'Sullivan show the power of to shape public policy.?

The writings of John L. O'Sullivan, particularly his advocacy for Manifest Destiny, showcased the power of ideas in shaping public policy by framing territorial expansion as a moral and national imperative. His assertion that the United States was destined to expand across the continent galvanized public sentiment and influenced political leaders, creating a justification for policies like westward expansion and annexation. O'Sullivan's rhetoric helped to legitimize actions that would have profound implications for both domestic and foreign policy, underscoring the role of ideology in guiding governmental decisions.