Alfred Thayer Mahan was a Flag Officer in the US Navy.he was in favour of imperialism - he believed that US needed overseas colonies to provide military and naval bases to protect its overseas economic interests and trade routes,and in particular the US merchant marine in times of war.
Eugene Debs was a union leader and most prominent member of the Socialist Party of America.He was against imperialism,seeing it as capitalist exploitation of foreign peoples no different to capitalist exploitation of the working class in USA.