I think what he is trying to say is your portrayal of the US government as an imperial power is underminded by the fact they haven't conquered Canada, that a truly aggressive imperial power would conquer everything around it.
However there are different types of imperialism. The US is not the Roman Empire, but it is similar to the Athenan Empire which was a democracy and supported local oligarchs in other countries to create client states.
Now the Athenan Empire was hardly the worse political entity in history, the US likewise is far from the worse political entity in history. I just think all their talk of freedom, while being allied with undemocratic governments like the Saudi monarchy rubs people the wrong way.
But I do think some left wingers over emphasis the negatives with US, I don't think its doing anything that different from other countries with power did and I do think there have far worse powerful states in the past. What Belgium did the Congo is worse then what the US has done to other countries. Plus when China becomes the world's super power, its not like the world will become a nice happy place.
Other countries can't just blame the US for their problems, they have take responsibly for their own actions. Sure the US government supporting the Shah was wrong, but the US government is not responsible for the creation of the current Iranian regime. After the revolution in Iran, the Iranians could have created any government they wanted, they choose to create a theocratic regime.
Now this is a thoughtful, intelligent response.
The American model of imperialism is different from other historical empires in that it does not rest primarily on force, though that is an important part of it. Rather, the U.S. government coerces other countries into acting the way it wants through its dominance over international bodies like the UN, the IMF and the World Bank, and by flexing its economic muscles.
Look at Canada. The main reason the U.S. has such power over our government's actions is not its military might, but its economic strength. The United States is Canada's single largest market, and given that our economy is particularly export-driven, it's important to keep good relations with our southern neighbour. Those political considerations, along with easy profits, is why we keep the Alberta oil flowing across the border.
The reason leftists devote so much anger to the United States is because as the world superpower, its actions have the most impact. If China or Russia were acting in comparable ways, leftists would be criticizing them (as they did when, for example, the USSR invaded Czechoslovakia in '68). This is also why Noam Chomsky explains his focus on American foreign policy - because as an American, he is in a position to do something about the actions of his government. To portray principled criticism of government actions as kneejerk "America-bashing" is just a Republican talking point.
The idea that "countries should take responsibility for their own actions" sounds reasonable on the surface, but it ignores the fact that there are radical differences in opinion among a country's population. Some Iranians wanted a theocratic regime, sure, but others wanted a liberal-democratic, socialist or communist government. 1/3 of the German electorate in the 1930s voted for the Nazis, but that means 2/3 of Germans opposed the Nazis. The country that ended up falling to fascism had the strongest labour movement in Europe at the time, and things very easily could have been different. But to excuse things like the firebombing of Dresden by saying that Germans had to take responsibility for their actions is to say that all Germans were Nazis.
Imagine if a more powerful country invaded the United States, and there were a huge amounts of American civilian casualties. War supporters in the invading nation could easily say, "well, the United States supported Dictatorial Government X and launched an aggressive invasion of Country Y. They're a brutal imperialist government and Americans need to take responsibility for their own country." Such a sweeping statement ignores the fact that most Americans don't vote at all, and of those that vote a huge number do not support the country's aggressive imperialist foreign policy.