I'd recommend this:https://www.marxists.org/reference/archive/mao/selected-works/volume-1/mswv1_17.htm
The takeaway I got from this is that contradictions exist in every thing and the "unity of opposites" means that in any subject, its development (rise and fall), motion (up, down, spinning, forward…) and forms are caused by the unity of opposites, which are the contradictions both depending and opposing each other. The collision between contradictions is responsible for all those changes we see from the material world.
For instance, poverty and prosperity of society, poor and rich, educated and uneducated, men and women, straight and LGBTQ, Christians and and Muslims, Abrahamic religion and non-Abrahamic religions, religion in general and atheism, Republicans and Democrats, socialists and capitalists, capitalists and workers, patriots and traitors, nomadic tribes and agricultural villages, manufacturing capital and financial capital, globalists and nationalists, imperialists and nationalists, subjectivity and objectivity, ideas and materials and so on are all contradictions with each other that fit into the description of the unity of opposites.
So, a man is a man different from a woman, but both of them could be white, so they are different from a black man and a black woman. Yet they are all Christians, so they are different from Muslims. Yet they are all Americans, so they are different from Chinese. The unity of opposites exists everywhere to promote and stimulate the development or death of every subject through the collision between the contradictions.
The implication from Mao is that antagonism is one form, but not the *only* form, of the struggle between opposites.
>In human history, antagonism between classes exists as a particular manifestation of the struggle of opposites. Consider the contradiction between the exploiting and the exploited classes. Such contradictory classes coexist for a long time in the same society, be it slave society, feudal society or capitalist society, and they struggle with each other; but it is not until the contradiction between the two classes develops to a certain stage that it assumes the form of open antagonism and develops into revolution. The same holds for the transformation of peace into war in class society.
>However, we must make a concrete study of the circumstances of each specific struggle of opposites and should not arbitrarily apply the formula discussed above to everything. Contradiction and struggle are universal and absolute, but the methods of resolving contradictions, that is, the forms of struggle, differ according to the differences in the nature of the contradictions. Some contradictions are characterized by open antagonism, others are not. In accordance with the concrete development of things, some contradictions which were originally non-antagonistic develop into antagonistic ones, while others which were originally antagonistic develop into non-antagonistic ones.
>Lenin said, "Antagonism and contradiction are not at all one and the same. Under socialism, the first will disappear, the second will remain." That is to say, antagonism is one form, but not the only form, of the struggle of opposites; the formula of antagonism cannot be arbitrarily applied everywhere.
Mearsheimer would disagree, but Mearsheimer represents the bourgeois ideology of the United States. There are also implications between socialist states. There have been antagonisms between socialist states. But those antagonisms can also turn into non-antagonistic contradictions, hopefully. An example today is China and Vietnam. There are definitely contradictions between China and Vietnam – they have a long history of antagonism. But what we haven't seen is Vietnam joining the U.S. alliance structure – they're deliberately rebuffing U.S. attempts to court them, and avoiding any military alliances, including with China.
But they do trade with China. One way to deal with the resources is to use this example: you have something I want, and I have something you want. What is the logical thing to do in this situation? We can attack each other and fight for these resources… or… trade. If we're both socialists and we have different socialist countries, I think the more logical thing to do is to trade with each other. We work together. Even if we're not necessarily "best buds." But that might be a way to turn antagonistic contradictions into non-antagonistic contradictions.