20th century

The 20th century, defined in the calendar of Earth as the period from 1901 to 2000, was a violent and tense time on Earth, with major conflicts like World War I, World War II, the Cold War and the Eugenics Wars. The dominance gained by the United States of America during these wars would prove to be a crucial circumstance in the prehistory of the Federation.

Decades

The National Aeronautics and Space Administration is founded on Earth in 1958

Major events

1903: At Kitty Hawk on Earth, the Wright brothers conduct the first ever powered manned flight by Humans. (ENT opening credit sequence; ENT: "North Star")
1915: Albert Einstein devises his Theory of General Relativity, a cornerstone for Humans developing technologies like the transporter and warp drive in the future. (TNG: "The Nth Degree")
1930s: A Civil war breaks out on the planet Xindus, which lasted until the 2030s. (ENT: "The Shipment")
1945: World War II ends with the United States of America being among the victors. The participation of the USA and its dominance over the following decades leads to developments resonating on into the 24th century. (TOS: "The City on the Edge of Forever"; ENT: "Storm Front, Part II"; TNG: "Manhunt")
1947: A Ferengi shuttle from the 24th century crashes on Earth near Roswell, leading to unofficial first contact between Humans and Ferengi. The "aliens" eventually escape and the event is covered up by the USA. (DS9: "Little Green Men")
1950s: After their first contact earlier this century, a border dispute begins between Andoria and Vulcan which lasts for the next two hundred years. (ENT: "Proving Ground")
1957: Sputnik becomes the first Human-made object to enter space. An investigating Vulcan ship crash-lands on Earth near Carbon Creek, Pennsylvania, leading to unofficial first contact between Humans and Vulcans. (ENT: "Carbon Creek")
1968: After Gary Seven secretly sabotages the launch of an American orbital nuclear weapons platform, a new and stronger agreement against the use of such weapons is made. (TOS: "Assignment: Earth")
1969: The Earth spacecraft Apollo 11 lands on the Moon, recording Humanity's first steps on another world. (TOS: "Tomorrow is Yesterday"; VOY: "11:59", "Nightingale", "One Small Step")
1996: Having cost the lives of over thirty million Humans, the Eugenics Wars, brought about in 1993 by a group of genetically-engineered "supermen" lead by Khan Noonien Singh, come to an end with their defeat. Khan and over fifty of his followers leave Earth aboard the SS Botany Bay. (TOS: "Space Seed"; Star Trek II: The Wrath of Khan; ENT: "The Augments")

Events without year specification

First contact occurs between Vulcans and Andorians during the early 20th century. According to the Vulcans, it "seemed promising," "despite their heightened emotions." However, it was soon discovered that "they were duplicitous," wishing to only honor "agreements that didn't conflict with their interests." (ENT: "Proving Ground")
According to Q, in this century, nations on Earth are fighting each other for territorial gains. (TNG: "Encounter at Farpoint")
Embarking a new era of technological warfare, World War I is fought on Earth during the early 20th century. (TOS: "Patterns of Force"; ENT: "Storm Front, Part II")
The later half of the 20th century is dominated by a cold war as well as several small conflicts in Asia called the Brush Wars. (TOS: "The City on the Edge of Forever", "A Private Little War")
The Denobulans start to use genetic engineering to "generally positive effect". (ENT: "Borderland")
In Human games, the penalty box is introduced. (TNG: "Hide and Q")
Music styles that originated in this century include jazz, crooning, and rock and roll. Composers and musicians include John Philip Sousa, Irving Berlin, and Gustav Mahler
While film recordings were pioneered in the 19th century, the 20th century sees the first major movie productions including The Andromeda Strain, The Black Cat, Frankenstein, The Day the Earth Stood Still, and later the first televised broadcast including series like I Love Lucy, Mission: Impossible, The Three Stooges, and The Twilight Zone.
In the 20th century cars, tanks, aircraft, and helicopters become commonly used.
In the later 20th century, France is a founding member of the European Space Agency. (ENT: "First Flight")