When looking at motive, you can look at two things: subjective intent and objective outcomes. It's hard to get a sense of subjective intent, but the starting point would be to look at stated policy goals. Of course, state actors often act contrary to their stated goals and have ulterior motives. So then you can look at objective outcomes: what happened to the areas affected by US imperialism? Do you think those outcomes were good or bad? And how do they fit in with the subjective intent or stated goals? Do they support them or do they seem contrary?