I believe that Manifest Destiny was the nineteenth century idea that the United States of American—led by Anglo-Americans—was picking up the mantel of world leadership and the white man’s burden from the British Empire and would become greater than its predecessor. I think it was an idea that began decades before its formal declaration, and continues in some diminished way to the present.
I think that Manifest Destiny was not about white Greeks and Bohunks, Irishmen, and Swedes. I think that “white” didn’t become a standard classification to include all Americans of European ancestry until after WW 2, when Bohemians and Swedes, Greeks, Italians, and Irishmen all served together. Until then—even through the Dutch-American Roosevelts, Anglo-Americans were the ideal, and the story of Manifest Destiny their story of crusading against and bringing Civilization to a vast wilderness. (Which of course leads to totally ambivalent attitudes towards American Indians—but that is another story.)
If you look at one factor only, the Read The Article