This whole thing is silly but it does point to the fact that Marx and Engels did support socialist imperialism for the American bourgeoisie at times, although their positions did change. I think it's a blemish on their record to be honest and some scholars suggest they later abandon it once Marx really studied American primitive accumulation in Capital Vol I.

But the logic of their support should be understood. Here is Engels on the American conquest of Mexico in 1847 for example. 

The second screenshot, from the same article "The Movements of 1847" will give you an idea of the higher logic: it has to do with the perception that within the bourgeoisie there is a militant revolutionary possibility that can reverse the movement of imperialism for the proletariat. It also has to do with the perceived necessity to complete the bourgeois revolutions.