Soviet Union was born in crisis, and had then gone from one disaster to another.
At the beginning, Soviets lost the south-east of Siberia, Central Asia, Poland, Ukraine, Belarus, Baltic countries and Finland. As soon as 1919, there were three anti-Bolshevik interventions, led by the West, the Draka and Japan.
The Russian Civil War opposed the Reds to various factions (Monarchists, Constitutional-Democrats, Mensheviks/Right SRs, Anarchists, unaligned warlords), supported by either the West, Japan or the Domination. The Western intervention was limited (occupying Kamtchatka, supporting Finland and Poland, protecting Caucasus). Westerners didn't have the will for a full-scale war in Russia, their main reason to be there was to contain the Domination, Japan and their puppets...
The Reds only won because their enemies were divided (their foreign patrons playing proxy war), seen as foreign agents, and even more brutal and unpopular than themselves.
Foreign intervention stopped in 1923. Poland created the Commonwealth of Warsaw, Finland annexed Karelia, and both threw the Whites under the bus, in exchange for Soviet recognition of their gains. The Draka couldn't sustain a war in Russia anymore, not while transitioning to autarky (after the split with Britain) and "pacifying" the Middle East. Japan was forced to stop by the Kanto earthquake.
The West was exhausted by World War I, and secretly happy to pull out of Russia. As soon as the Draka and Japanese pulled out, the Westerners followed.
That allowed the Red Army to win for good in 1925 against all White factions. Despite the Whites receiving foreign weapons.
However, Russia was in a desperate state. It had been at war for nine years. Since 1918, there had been the Red Famine, caused by food requisitions, looting, displacement of population and scorched earth tactics. There was also pandemics. Not only the Spanish Flu, but the return of cholera, typhus, plague and other diseases.
And finally, there were the Green militias, made up of rebel peasants, opposed to both Whites and Reds. The Soviet government had to maintain war communism, to rebuild the nation and to develop the militaro-industrial complex (to stand up to Draka and other threats). The Greens were really fed up of the Reds. No compromise could be found.
The result was that, instead of focusing immediately on fighting hunger, disease and homelessness, the Soviet government spent three years methodically eradicating Green militias, using organized famine, chemical and biological weapons, and air bombing (with balloons).
At the end of 1928, Soviet Union had lost between 27 and 63 millions of lives. And hadn't really began to rebuild yet.
As dust finally settled, Stalin initiated his policy of forced industrialization and collectivization, leading to the Famine of 1933 and the Second Green Rebellion. The Red Army didn't have much will to fight the Greens (nor success). Economic success wasn't there either.
So, Stalin convinced himself that Russia was full of traitors and saboteurs, leading to the Purges of 1935-1941. Of course, those purges only made the political and economical situation worse.
In 1936, the Second Civil War officially began, with a coup led by Tukatchevsky (who was about to be purged). He created the "Salvation Council". Salvation forces were finally beaten in 1938, while the Purges continued. After the Second Civil War, the Red Army was put under complete control of the NKVD and the Red Guard.
The invasion of Ukraine, Belarus and Baltic countries saw the Soviets bogged down against a fanatical guerilla (joined by many Russian defectors). The Winter War further fragilized Russia and its government.
Russia would exit the Interwar technologically, scientifically and militarily backwards, deprived of competent cadres, and utterly hating its own government.
The only project that succeeded was the Voroshilov Line, which protected the Draka border in Central Asia (emulating the Maginot Line). Building, maintaining and manning the Line costed the lives of millions of workers, and forced Stalin to neglect almost everything else, but it was done.
After his non-aggression treaties with Germany and Japan (1939), Stalin also massed the bulk of Soviet armored corps, elite forces and air forces in the south, near the Draka border. This would be his one good decision during his rule.