Captains Log: The Enterprise is on its way to Penthara IV where a type C asteroid has struck an unpopulated continent. The resulting dust cloud could create a phenomenon not unlike the neuclear winters of 21st century earth.Source
Probably the most significant action of the 1900's was the voluntary ending of the "dictatorship of the proletariat" by the people of the USSR (the Soviet Union) and responsible people throughout the world. This action ended the cold war standoff with the capitalists which brought with it the constant threat of nuclear holocaust. It also, according to capitalist pundits, collapsed the Soviet Union and the Warsaw Pact organization. The main flaw in this "collapse" theory is that you cannot collapse people. All the people in post USSR and in the Warsaw Pack are still there and they are fully aware of what socialism is like and are perfectly capable of comparing what they got from it with what they are now getting from capitalism. Capitalists will find that in the end, they would have been much better off to have left well enough alone and to have left the "iron curtain" in place; they will find that the people themselves both Russia and the other former Soviet countries as well as those in the former Warsaw Pact countries will do much to help spread the "good news" of socialism and speed the socialist revolution throughout the world.
The three key events for the socialist revolution in the 1900's: 1) the people correctly chose Lenin to successfully begin the socialist revolution; 2) the people correctly back Stalin to protect the socialist revolution against capitalist reactionaries (the fascists and their collaborators); 3) the people correctly agree with Gorbachev that the time had come to initiate the next state of the socialist revolution: the period of people's republics (the construction period of socialism).