They wanted the U.S to help Cuba and then take it. Manifest Destiny was part of it. they believed that it was God's will for them to expand. alot like when they traveled to the west past the Appalachian mountains.
They also argued that it will help the Nation with sugar trade and other things. Simply putting it, they wanted to make colonies and expand. <- all i got. sorry.
This actually belongs in history. The Era of Imperialism started in the later part of the 19th century overall, though you can look at England's claims in the Congress of Vienna as the first real moves of an Imperialistic policy.
Anyway, long answer. Pro imperialism (no dash needed) refers to a person who is favorable to imperialism.