In the 1860's, the USA had already gained so much territory it pretty much owned all of its current territory on the American continent (except for the scarcely populated Alaska).

Personally, I believe the Louisiana Purchase to be of the greatest importance. Without it, the USA could never have had a Western frontier, would never have reached the Pacific and become an important power in that ocean. The growing conflict between Free States and Slave States would have evolved very differently if it would not have been for the frontier, the annexation of new territories and political disagreement on extending slavery to new territories or not. Even if the Union woudl have won the War Between the States, the USA would not have been a Super Power if the Mississippi would have been its western border.

Would Louisiana have become American anyway? We don't know, but history would have been very different anyway.

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[U]President Josiah Bartlet:[/U] Sweden has a 100% literacy rate. 100%! How do they do that?
[U]Leo McGarry:[/U] Maybe they don't and they can't add.