Making it into a pseudointellectual argument (i.e. "you're using a fallacy") also really helps liberals discredit it. Ideologically, American liberalism styles itself as a technocracy; the political class and those who support them are simply more understanding about these issues than the common person. It's why you'll see libs parrot MSNBC talking points about why we can't do this or that radical thing. They see themselves as more qualified to be making decisions than others and are therefore susceptible to arguments which stroke that ego about being one of the smart ones. By making the counter "you're using a logical fallacy" libs can say to themselves "oh its one of the uneducated, since they can't argue properly. I don't need to listen to them or question if this reaction is appropriate because I'm so much smarter and more politically knowledgeable than them"
I don't know if you can, sadly. I've tried to explain to liberals that conversations like these exist within an implict political context, that criticizing the most successful socialist project as inherently immoral is criticizing socialism in general by proxy. No one outside of the left (and honestly not even everyone within the left) are able to escape the propaganda chain of USSR = communism; USSR = bad; communism = bad. We know that's what's happening here, but I often find trying to point that out is met with this obstinate response of "I'm just talking about the USSR".
I've also tried explaining that even more broadly, this standard never gets applied to the imperial core. We don't discourse about how the United State's treatment of queer people during the AIDS Crisis was a literal genocide and then extrapolate that onto the morality of the US. The issue there, again, is that layers of other excuses and propaganda kick in.
The real answer is that we don't try to convince liberals that imperialism is bad. We convince liberals that their own personal material conditions are bad as a result of the actions of capitalists, slowly get them to realize what regulatory capture is, and then try to deprogram their brainworms about American imperialism. It's frustrating, but it's much harder to propagandize away someone's material suffering than the actions of empire on the periphery. We make liberals leftists, then we educate baby leftists on imperialism and so on