Perhaps the most fateful consequence of the Great War was the Russian Revolution, which led to the establishment of the world's first socialist state. It was an event that raised many hopes for a better future—hopes that would be dashed as the Union of Soviet Socialist Republics turned onto the path of totalitarianism. Thereafter civil war, famine, purge, repression and massacre convulsed the lands of the former Russian Empire. But hope died hard, and even at the height of the Stalinist terror there were people on the Left in Europe and America who looked to the USSR as humanity's savior, leading the world toward a radiant socialist future. In fact, however, the voluntary union of sovereign socialist republics established in 1922 was a facade behind which the ruling Bolshevik Party recreated the tsarist empire in a new form. Lenin and Stalin after him were determined to build socialism from the center, and their gestures in the direction of federalism and ethnic autonomy were strictly tactical. When nationalism was thought to threaten Party control, it was ruthlessly stamped out. Ukraine in particular, with its language and culture distinct from Russia and its genuine national consciousness, was viewed by the Bolsheviks with suspicion and hatred. Stalin's solution was the terror famine of 1930-33. Throughout the USSR, the collectivization of Soviet agriculture was carried out by force—and in Ukraine force was applied with particular savagery, in a calculated attempt to stamp out the Ukrainian national idea. The resulting famine killed nearly four million Ukrainians, and it was followed by a vicious purge of the Ukrainian intelligentsia.

In form the USSR was a union of sovereign republics. There were four of these in 1922 when the union was established: Russia, Byelorussia, Ukraine and Transcaucasia. At the time of the USSR's dissolution there were fourteen soviet socialist republics (SSRs). the fifteenth, Russia, was unique among them in that it was itself federated to accommodate various smaller national minorities in a series of autonomous soviet socialist republics (ASSRs). The Transcaucasian federated republic was dissolved in 1936, its three autonomous republics becoming union republics. The Karelo-Finnish SSR, established in 1940 by adding former Finnish territory to the existing Karelian Autonomous Soviet Socialist Republic, was probably intended to encompass the whole of Finland. But that country was able to maintain its independence after World War II, leaving the KFSSR as something of an anomaly within the USSR—in fact, only about 10% of its population was ethnic Finnish. So in 1956 it reverted to the status of an autonomous republic of the Russian Soviet Federated Socialist Republic.

The red flags of revolutionary Russia and the USSR perpetuated the heritage of the Red Banner, since the time of the Paris Commune the primary symbol of revolution and socialism. Where dates are given below, these indicate the period of time during which the flag depicted was in official use.
 

First National Flags of the Russian Soviet Federated Socialist Republic

Byelorussian SSR  •  1919-27

Following the Bolshevik coup in October 1918 a plain red flag—the Red Banner—was adopted as the de facto national flag of revolutionary Russia. Upon the formal establishment of the Russian Soviet Federated Socialist Republic (RFSFR) the initials of the state, in golden yellow Cyrillic characters, were added and this flag, of which there were several variants, was officially confirmed. The early red flags of the other socialist states that would constitute the Union of Soviet Socialist Republics (USSR) mostly followed this pattern. The flags depicted above were those of the four founding  union republics of the USSR. The Transcaucasian Socialist Federative Soviet Republic embodied the autonomous republics of Armenia, Azerbaijan and Georgia. It was dissolved in 1936 and its members became separate union republics.

The first flag adopted upon the formation of the USSR displayed the new state's coat of arms on a red field. After only four months, however, it was replaced by a simpler design: red with  the symbols of socialism and communism—red star, worker's hammer, peasant's sickle—placed in the upper hoist. The star was bordered with golden yellow and the other two symbols were solid golden yellow. The flag decree specified that the symbols were only to appear on the obverse of the flag, leaving the reverse plain red, but it was often made with the symbols on both sides. Over the years, minor alterations were made to the artistic rendition of the symbols on the flag of the USSR but otherwise it was not changed between 1922 and the dissolution of the union in 1991.

Many of the USSR's ministries and official organs had distinctive flags, sometimes variants of the national flag and sometimes of a unique design. Two examples are shown above.

The first flags of the union republics were red, mostly with the name or initials of the republic in golden yellow and sometimes with the star, hammer and sickle. Their final flags were adopted from 1950 onward. When the United Nations was formed the USSR as a whole plus the Byelorussian and Ukrainian SSRs were given seats in the General Assembly. Because the existing flags of the three were so similar in design, the UN demanded changes. Ukraine thereupon adopted a red flag with a broad light blue stripe running from lower hoist to lower fly—blue being a color long associated with Ukraine. The star, hammer and sickle were placed in the red field. Byelorussia's new flag was red with a vertical panel at the hoist depicting a pattern of traditional Byelorussian embroidery, a green stripe running from the vertical panel to the fly, and the star, hammer and sickle in the red field. By 1960 all of the union republics had adopted similar flags with design features reflecting the republic's national character. For example, Estonia and Latvia, situated on the Baltic Sea, had flags with a design of waves. The white and green stripes on the flag of the Tajik SSR symbolized cotton, the state's most important crop, and agriculture in general. The flag of the Georgian SSR was exceptional in being the only one not depicturing the star, hammer and sickle in golden yellow. As with the flag of the USSR, the star, hammer and sickle were supposed to appear on the obverse only, but this stipulation was frequently ignored.

Along with the flag of the USSR itself, those of the union republics were discarded upon the dissolution of the union in 1991. In many of the now-independent republics, it is illegal to display the flags of the communist era, for example in Ukraine.

The ASSRs within the Russian SFSR had flags identical to that of the RSFSR with the addition of the state's name or initials. Two examples are shown above.