Patriotism has nothing to do with "being proud" of anything. Were the CPC and the KMT proud of anything when they started the reunification wars? Ofc not, and thats exactly why they started those wars. Patriotism is about loving your country, and, despite what fake patriots (meaning the bourgeoisie) would want you to believe, your country is made up of its people, its not an abstract idealistic idea of "America" or any other country. China was in shambles, and it was the CPCs patriotic duty to rebuild it and save the chinese people.
This idea that "you cant be patriotic of imperial core countries" and that if you do you are proimperialist is ridiculous and doesnt exist outside of the USA. If you dont believe me, read the party programmes of european communist parties and what they have to say. A good example is the Communist Party of Greece (KKE), an ML antirevisionist and very big party that led the fight against the austerity imposed on Greece by the EU after the 2008 crisis. The KKE openly says that they are patriots, and that its their patriotic duty to oppose the EU and its global imperialism. Most european communist parties say the same thing, and the few that dont tend to be the most proiimperialist liberal and eurocommunist ones.
You americans keep repeating this lie, but its false, and all you have to do is look outside your borders to see im right. Idk why you are so obsessed with this idea, maybe its a reaction to the toxic jingoism you have over there, which calls itself "patriotism" but is not. There is nothing patriotic about joining the US military and drone striking afghan children. In fact, i would say its the opposite, its every americans patriotic duty to NOT join the US military, since the only thing that allows US capitalists to mantain their power is having enough cannon fodder to fight for them in the military.
Also i dont understand what your last sentence is about, i said nothing about nationalism.
To add on my previous response, i think this quote by Mao Zedong from the time of the japanese invasion of China is quite relevant on this:
"Can a Communist, who is an internationalist, at the same time be a patriot? We hold that he not only can be but also must be. The specific content of patriotism is determined by historical conditions. There is the "patriotism" of the Japanese aggressors and of Hitler, and there is our patriotism. Communists must resolutely oppose the "patriotism" of the Japanese aggressors and of Hitler. The Communists of Japan and Germany are defeatists with regard to the wars being waged by their countries. To bring about the defeat of the Japanese aggressors and of Hitler by every possible means is in the interests of the Japanese and the German people, and the more complete the defeat the better. For the wars launched by the Japanese aggressors and Hitler are harming the people at home as well as the people of the world.
China's case, however, is different, because she is the victim of aggression. Chinese Communists must therefore combine patriotism with internationalism. We are at once internationalists and patriots, and our slogan is, "Fight to defend the motherland against the aggressors." For us defeatism is a crime and to strive for victory in the War of Resistance is an inescapable duty. For only by fighting in defense of the motherland can we defeat the aggressors and achieve national liberation. And only by achieving national liberation will it be possible for the proletariat and other working people to achieve their own emancipation. The victory of China and the defeat of the invading imperialists will help the people of other countries. Thus in wars of national liberation patriotism is applied internationalism."
Source: https://www.marxists.org/reference/archive/mao/selected-works/volume-2/mswv2_10.htm
As you see, Mao clearly states, as indicated by the "" around the word "patriotism", that the so called "patriotism" of the imperialists is false, that not only do the invaded and oppressed have the patriotic duty to fight back, but also the working class of the invader countries has the patriotic duty to fight back against their own government, since the war doesnt benefit them either. Thus, as Mao states, "[communists] not only can be [patriots], but also must be [patriots]" and this is fully compatible with internationalism. And this makes sense, the workers of the world have the same interest, to achieve socialism and then communism, so it only follows that by fighting for the interests of your countrys workers, thus being a patriot, you are also fighting for the interests of the workers of the whole world, thus being an internationalist.