Marxist predictions of a revolution in a wealthy capitalistic society failed to materialize; the "proletariat" was enjoying an increasing standard of living in Britain and Germany, where these revolutions were supposed to have occurred, which really upset and confused the devout Marxists.

So, Marxists began to split into factions, which tried to revise or reinterpret Marxist teachings to include nationalism (rather than classism) and to include direct violent upheaval rather than passive waiting. In Italy, there were the rise of the syndicated "unions", or "fascists", where the struggle became between not rich and poor people, but between rich and poor countries. This idea of nationalistic socialism spread to Germany, which added to it the idea that there is a struggle between the races, too.

Why did the Nazis so hate the ? Because the communists were rivals for the Nazis socialist powerbase; all of this stuff is leftism

In Europe, the people on the "right" were the monarchists, who wanted to conserve the old order.

In America, the people on the "right" are the people who want to conserve the ideals of the American Revolution, in the face of the unending onslaught from leftist "progressives"; the "right" are the true Americans, who want a small government whose sole role is to protect the rights of the people (notice, the government doesn't grant rights, but rather protects rights), whereas the leftists want to increase power ever more in the centralized state, and either spread their control to the rest of the world (kind of a communist outlook) or simply nationalize large swaths of society (kind of like the fascists of Italy and Germany).

Either way, the point is that Marxists, Communists, Leninists, Trotskyists, Stalinists, Nazis, Socialists, Progressives, Antifa, etc., are all Leftist political movements.