Imperialism isn’t something that exists as a static concept, but functions differently depending on the dominant mode of production. The US Empire absolutely fits the Marxist understanding of imperialism as a specific stage of late-monopoly capitalism.

Imperialism as a concept predates Marxism and isn’t reducible to Lenin’s model. We can debate which framework is more useful, but pretending there’s only one definition isn’t serious.

The processes of earlier forms of imperialism predate Marxism, such as Roman imperialism. The analysis of capitalist imperialism, on the other hand, is most well-understood by how Lenin analyzed it with Marxism. Lenin wasn’t invalidating earlier forms of imperialism, but analyzing the specific character of capitalist imperialism, the form that by far matters the most today.

Lenin’s framework is one influential analysis of capitalist imperialism. That doesn’t make it exhaustive. Modern geopolitics also includes state security competition, regional spheres of influence, and non-capitalist power projection.

Marxists have also continued to expand analysis of imperialism beyond Lenin. One such example is Cheng Enfu’s analysis of neoimperialism, where imperialist countries have ralied behind a single dominant Empire, the US, rather than compete with each other (though this is falling apart now). Geopolitics isn’t limited to imperialism, but imperialism is the principle contradiction driving development in the world today, that being the socialization of global labor struggling against the privatization of the profits made by global labor in the hands of the few in imperialist countries.

Calling imperialism the principal contradiction is a theoretical commitment, not an empirical conclusion. Other schools like realism or institutionalism would identify state security competition or balance-of-power dynamics as primary.

And what are the opposing tendencies in these contradictions?

In realism, the opposing tendencies are expansion of one state’s power and balancing by others to preserve sovereignty. In institutionalism, it’s integration versus fragmentation. Neither requires framing global politics as capital versus labor.