The October Revolution that brought the Bolsheviks to power in Russia occurred on October 25 (old style), 1917.
As it happens I have recently finished reading a book by Orlando Figes, A People's Tragedy: The Russian Revolution: 1891-1924, which as the title indicates, covers not just the 1917 revolutions, but the period leading up to them, and the complicated civil war that followed them. While most people know about 1917, having the whole context helps a lot in understanding those events. Follow the squiggle for an overview (based largely on Figes's book) and some comments.
Russia in the early years of the 20th century was still largely an agricultural society. In central Russia, the peasant population had a communual land tenure system. They divided the land into small strips and each year each family was assigned a different strip. This was an inefficient system and it gave little incentive for peasants to improve the land they were allocated, since they weren't going to stay on it. But it seen as fair, and the peasants were very attached to it (this system however was not used everywhere in the Russian Empire).
The peasants had village councils that were usually dominated by a few village elders. Those elders could be stubborn and resistant to change. There were petty jealousies and feuds in the villages. But the peasants valued self-rule and local democracy. In particular they mistrusted outsiders and didn't want interference in their local affairs.
While serfdom had been abolished, still title to the land was generally held by local aristocrats who allocated some of it to the peasants and had kept the best for themselves. The landowner could compel the peasants to work the land he owned. The peasants resented this. They were not very receptive to abstract arguments about class struggle and the like. But they knew who was oppressing them: it was the landlord. However, there was very little support from the government for any reform or change to this system.
One of the critical years was 1905, when, in the aftermath of the disastrous war with Japan, a revolt against the Tsar took place. It was put down, with considerable loss of life, and the brutality of the Tsarist forces did a lot to weaken popular support for the Tsar. But concessions were also made, especially the establishment of an elected body, the Duma.
The Duma had representives of the aristocracy (some of whom actually had liberal inclinations), but aside from those there were a variety of parties representing the common people. The Bolsheviks were a minority among these. The Social Revolutionaries and the Mensheviks were among the larger parties.
The Mensheviks believed in a socialist future, but they subscribed to the Marxist theory that Russia must first go through a capitalist phase and only then could it involve beyond that into socialism. For this reason they supported a consitutional democracy as an interim step towards the socialist utopia. The Social Revolutionaries had a spectrum of viewpoints, but general also supported a democratic socialist system. But at the same time, they were quite militant, and had a "combat wing" that carried out assassinations. They had widespread support in the peasant population, due to their advocacy of land reform. The Bolsheviks favored a radical abolition of private property. They also believed that a revolution could be achieved without going through a transitional phase of bourgeois capitalism, and that it could be led by a "vanguard" of dedicated party members.
The Tsar however resisted any real reforms. His father had been an absolute monarch and he was determined to rule as one, too. He viewed the Duma as an unfortunate concession, and was not going to yield to its demands.
The Great War that begin in 1914 really completed the destruction of the monarchy. Russia was unprepared for it. The war ground on, long past the point when the participants believed it would be over, and it was tremendously destructive. The Russians were getting the worst of it. Desertions were increasing. The Tsar himself assumed command of the armed forces, a role he was extremely ill-prepared to fill. He left the civil government largely in charge of his wife and her chief advisor, Rasputin, an almost illiterate peasant monk.
The first revolution in 1917, the February Revolution, swept the Tsar out of power and installed a parliamentary democracy. This was called the Provisional Government because the plan was to later hold elections and create a properly elected government.
But the Provisional Government was destined to be short-lived. One crucial fault was that while they had removed the Tsar, a majority was in favor of continuing the war, a very unpopular stance. Basically, they had a Tsarist foreign policy without the Tsar.
On the domestic front, Figes talks a lot about the "revolution on the land." After the February Revolution, many peasants took advantage of the breakdown of government and the confusion of the war to seize land from the local gentry, and distribute it among themselves. For them, this was practically the only revolution that mattered. The Provisional Government installed after February never supported this, but did not have the power to reverse it. The Bolsheviks, however, did support it (although their actual preference was for nationalization of land and collective ownership). The Social Revolutionaries split over this issue and the war, with a "left" faction siding with the Bolsheviks.
Even before the February revolution, Soviets (local councils of workers and peasants) had been formed throughout the Russian territories. These were practically a parallel government, and offered a very direct version of democracy: locally chosen leaders making decisions on local affairs. For the peasants, they were very much like the village assemblies in pre-Revolutionary Russia. The Soviets had broad popular support, and the Bolsheviks built on this: they adopted the slogan "all power to the Soviets!," which implied they were going to make the Soviets the true government of the country. But that was not to be. Figes says possibly the Petrograd Soviet could have seized power, and headed off the October Revolution that brought in Lenin and the Bolsheviks. But they had no leaders willing to take this step. Trotsky was a prominent member of the Petrograd Soviet, but finally in August 1917 he officially joined the Bolsheviks and aligned himself with Lenin.
The Provisional Government was practically paralyzed by October, unable to find a compromise between the rightist factions which wanted to go slow on land reform and the war to go on, and the leftists who wanted immediate redistribution of land and an end to the war.
The Bolsheviks had a power base in Petrograd, the capital, especially among the industrial workers and the military. They were primarily a party of the urban working class. Many of their leaders, including Lenin, had scant knowlege of the peasantry, who were still the majority of the Russian population. When the Bolsheviks seized power in October 1917, they had little trouble arresting the Provisional Government ministers. But it took some months to establish themselves as a functional government even in Petrograd, and they knew it was going to be a hard struggle to rule the rest of the country. They were haunted by the knowledge that multiple previous revolutions had failed: notably the Paris Commune, which had captured the capital city of France, but ended with its leaders being lined up against a wall and shot.
After taking power, the Bolsheviks were still weak and for a while were willing to cooperate with their political rivals, especially the left SRs. They actually allowed elections to occur for the Constituent Assembly that was supposed to succeed the Provisional Government. And the Assembly met for an initial session in Jan. 1918 that stretched long into the night and the next morning. But then the Bolsheviks shut it down, and it never met again. The purging of their enemies soon began in earnest. Members of rival parties were arrested and jailed or executed, and many who survived went into exile. Their publications were suppressed. Eventually even the "left" SRs fell victim, especially after an abortive coup attempt, and the new Soviet Union
effectively became a one-party state.
The Soviets were curtailed. Already pre-October the Bolsheviks were strengthening their power in the Soviets. Increasingly, power was held and exercised by small committees, which the Bolsheviks made sure to dominate, even when they were not a majority in the Soviet at large.
During the Red Terror, there was also a fairly indiscriminate rounding up of all kinds of supposed enemies of the revolution. The official line was that "bourgeois" elements were targeted, but it was always unclear who qualified as "bourgeois," and people could be swept up into the Terror just by being denounced to the newly-formed Cheka by personal enemies. In the countryside, there was an effort to root out "kulaks," i.e., richer peasants who oppressed the labouring class. This was partly based on the theory that class divisions must appear among the peasantry, as a stage in the Marxist evolution of society. But this was largely an illusion. The peasants were oppressed, but mostly by aristocratic landowners and outsiders, not by their own kind.
However, by far the most prominent losers in the revolution were the aristocracy. Those who survived had a drastic fall in their fortunes. Well-bred ladies were selling their last possessions, and sometimes themselves, on the streets of Petrograd.
Some among the Bolsheviks (but not Lenin) actually wanted to continue fighting in WW I, but with the aim of spreading revolution into neighboring countries. Some had in mind the post-Revolutionary wars in France, which didn't ultimately export the French Revolution, but did for a while give France military gains. At least they wanted to stall for time in hopes their example would cause revolution would take hold in Germany and other countries. But they eventually had to sign the Treaty of Brest-Litovsk with Germany, which granted the Germans vast swathes of territory formerly controlled by Russia. The SRs left the government over this, but Lenin defended it as a grim necessity. Fortunately for the Russians, the Germans later lost the war, and the German gains from that treaty were undone, although the Soviets did not then regain all their former territory (the Baltic States in particular).
The Bosheviks never quite gave up the vision of worldwide revolution. They re-formed the Communist International and encouraged Communist uprisings abroad. But this was not considered an essential goal, without which the Revolution was incomplete (and in any case, Russia in 1918 lacked the ability to continue to fight foreign wars). Stalin was later to put forward the slogan "Socialism in one country," emphasizing that the first goal was to strengthen the Soviet state. But Trotsky, among others, never accepted this limiting of the Communist vision.
The civil war that followed the revolution was somewhat slow in starting, but eventually grew into a large-scale conflict. Both sides dragooned reluctant peasants into supporting their side, through forced conscription and by confiscation of food and supplies (which eventually contributed to a large-scale famine). Desertion was extremely common, despite harsh penalities meted out by the competing armies.
Both sides struggled to obtain popular support. Lenin's government had to deal with substantial anti-Bolshevik rebellions. Peasants resisted conscription and confiscation, and various national and ethnic groups tried to achieve independence, sometimes succeeding for a while (for example, in Ukraine). There was even in 1921 a mutiny of the sailors at the Kronstadt naval base, which had played a prominent role in supporting the October Revolution. The Bolsheviks had to storm the base, with thousands of casualties.
In general though, the White armies, led by ex-Tsarist officers, had a harder time gaining popular support, because they were seen as representing the hated aristocracy, and they opposed the fait accompli of land seizures. There was no going back to the Tsarist system, but they had nothing to offer the mass of people that was signficiantly different or better.
The Bolsheviks educated and indoctrinated the millions who served in the Red Army, even if temporarily, and this had some effect. The Whites were beset by internal bickering and competing agendas. While multiple foreign powers intervened in the conflict on the White side, eventually they cut their losses, realizing that they were backing a hopeless cause.
Some comments of my own:
The October Revolution is still controversial, and still divides people, much as the French Revolution did in its time. Indeed, Figes's book is but one interpretation of that revolution. For decades, the Soviets defended every action of the revolution and of its leaders, however repressive or violent, and put forth a party-approved version of all its major events. And from an early date, there were other, dissident accounts as well, notably Trotsky's own opinionated book.
Figes is critical of many aspects of the Revolution. And certainly the revolution and the civil war that followed were hugely destructive. Still, the Bolsheviks did not instigate it just to take power for themselves. They were dedicated revolutionaries who wanted to re-make society, and in a way that would benefit the workers and peasants, people given short shift by the Tsar. And indeed the revolution had some positive achievements. It swept away the Tsarist system, which itself was very repressive. It extracted Russia from a horrible and unpopular war. It brought thousands of people from modest backgrounds into the government, the officer corps of the army, and other institutions from which they had been largely excluded. The hated aristocrats were evicted from their land, and it was redistributed. The early Soviets also greatly expanded education, and accelerated the transition to an industrial economy.
Many both inside and outside of Russia were inspired by the Revolution, especially in its early years.
But the legacy of the Revolution is a mixed one. Some still view Lenin in a more positive light than his successor, Stalin. Figes argues that practically every crucial feature of the Soviet system was in place at the end of the Russian Civil War: the co-option of the Soviets themselves into a kind of show of democracy without the substance; the rounding up of dissidents, even those on the left; the secret police, and so on. Still, Lenin was a more flexible leader than Stalin. At several crucial points he was willing to change tacks and try something different, even if it violated Communist orthodoxy. Too, as Figes notes, while he could be brutal to political opponents, Lenin didn't murder his Bolshevik comrades, while Stalin's show trials in the 1930s sent even loyal and long-tenured Bolsheviks to the firing squad. (Trotksy was driven into exile, and founded an anti-Stalinist Communist movement, before being murdered by Stalin's agents in 1940). The show trials, and the inexcusible Nazi-Soviet pact of 1939, drove many foreign adherents away from support for the Russian Communist Party. But that is another phase of history, one that is largely outside of Figes's book.