The theoretical founder of socialism was Karl Marx (1818-1883).
The most brilliant expression of 20th-century Marxism was Leon Trotsky (1879-1940).
True socialists have read & absorbed these books. Pseudo-socialists don’t read anything serious.
Socialists understand the USSR was Stalinism, which is collectivist property relations controlled by a fascist bureaucracy. Trotsky fought ceaselessly against Stalinism, and was historically vindicated in December 1991, when the Soviet Union collapsed back into capitalist Russia.
If Russia hadn’t had a workers’ revolution led by Lenin & Trotsky in 1917, it would be completely dismembered (balkanized) today.
Instead, modern Russia retains much of it’s size, and possesses the second-largest nuclear arsenal in the world.
A third world war would end human civilization. Today’s thermonuclear devices are orders-of-magnitude greater than the atomic bombs dropped on Hiroshima & Nagasaki at the end of World War 2.
Human society can only be preserved (meaning a world war can only be prevented), by a permanent socialist revolution led by the workers & youth of the U.S. The epicentre of global capitalism (culturally, economically & militarily) is the United States; a socialist revolution here leads the way for the rest of the world.
Marxist analysis in the 21st century is found at the World Socialist Web Site— wsws.org. The WSWS is published (6 days a week, since 1998) by the International Committee for the Fourth International (ICFI), the party founded by Trotsky in 1938.
The U.S. branch of the ICFI is the Socialist Equality Party (SEP). The SEP announced its candidates for U.S. president & vice-president on April 22– Earth day. Ric Size officially endorses the Socialist Equality Party’s presidential campaign. 
To those interested in actual socialism, this is the only choice. If you would like to be a socialist, you must put in the work. Every other leftist political party, from the Democrats & Greens to the anarchists, are opportunist– meaning they are capitalist to the core. Marxists always spot them quickly, and easily dispatch them when engaged in polemics. That’s why Trotskyists are always blacklisted from ‘official’ discourse.
Those who are politically naive, but well-meaning, need to work through this as quickly as possible. Read the books shown, along with the WSWS on a daily basis to arm yourself politically.