Which nation was caught off guard by the aggression of the
United States during the war that
liberated Cuba?

Spain

Explanation:

Spain had been the ruler of Cuba and the Philippians and desperately wanted to hold onto Cuba because this was their last major colony in the Americas and reminded them of the glorious and powerful Spanish empire of years past.

They did not however, anticipate the strong support the U.S. had for the people of Cuba who were seeking independence from Spain. When the USS Maine (1889) was blown up in Cuba, the U.S. used this as justification to go to war with, and defeat the Spanish who were shocked at how fast things escalated.

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