For more than a century, Americans had followed George Washington’s advice to stay out of European “entanglements.” Isolationism was America’s default foreign policy, and it had worked to our advantage. Additionally, for reasons of heritage, many big immigrant communities (Irish, German, Jewish) supported the Central Powers. Unable yet to vote, women demonstrated against sending “our boys” to a bloody foreign war. Socialists of all stripes rejected imperialist war altogether as did Wilson’s Secretary of State. All that constituted a formidable bloc of opposition to joining the war on the side of the Entente.
To overcome it, Wilson demonized the Germans, billing them a “Huns,” and buying into the list of war crimes the Entente had laid at their door. The Lucy’s sinking was the crown jewel in the Entente’s list of German war crimes and a such helped to push the United States into the First World War.
We now know that the Lusitania was a joint British/German war crime, not a German war crime. Thanks to the political cover-up on both sides of the Atlantic, the American people could not know that in 1915-17. But, had the true facts been in evidence then, the American people would have understood the Lusitania tragedy as confirmation of the wisdom of our policy of neutrality, not an argument for war.
Because other causes combined to encourage intervention, notably the our relentless arming of the British and consequent submarine warfare, no one can assert that the truth about the Lusitania would have prevented the United States from entering the Great War. But the truth would have shifted the Lusitania case from the war column to the neutrality column, and all its influence would have resisted (not encouraged) the war drums.