On April 21, 1898, the United States declared war against Spain. It would be the first overseas conflict fought by the U.S. It involved major campaigns in both Cuba and the Philippine Islands. ... Their quest was described as a "splendid little war" by Secretary of State John Hay
Yes, victory by the Union was inevitable regardless of how well the Confederacy fought or the intervention of any external potential allies.
Why? The North had a demographic advantage over the South - There were more people living the North than the South.
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