I ask because it seems to me, in that situation, most of the experienced military talent – the career officers, mostly of upper-class origin – would have gone over to the White side. How were the Bolsheviks able to win against that?
Several years of soul-crushing, exhausting war in the WWI trenches had something to do with it.
Also, the Whites had no local support. Given that foreign support could only be moved in during the Summer, when ports were not blocked with ice, local support was key. The Bolsheviks just had more Russians that liked them.
More importantly:
1.) The Reds had a much more unified command structure. While there was certainly infighting between factions, Lenin was in charge and no one was directly working against him in the Red faction. Conversely, the Whites had no single leader to either unify the cause or to keep White factions from fighting each other more than their opponents.
2.) The Reds had interior supply lines, the Whites did not. The Reds held most of Russia itself, and were beset by White forces from Siberia, the Caucasus, Poland, and the Baltic States. But those White forces had no way to coordinate and communicate with each other, allowing the Reds to move forces from one front to another to best respond to crises.
3.) An overwhelming number of brilliant, upper-class generals do not in and of themselves win a war. Ask the Confederacy.
There was also nothing resembling unity on the side of the Whites. You had monarchists, Social Democrats, Mensheviks, and all others sorts…generally everybody who hated the Bolsheviks. Trotsky, OTOH, had unity of command, ruthlessness and the will to win, and a goal. It was only a matter of time.
Also: the Red leaders did a brilliant job of telling each faction what it wanted to hear - promising independence to the Ukraine and Baltic States; hard-line Communism to the Bolsheviks (after the war NDP was over, of course); soft-line Communism and Socialism to the Mensheviks (just like the war NDP is, comrade!); etc.
The White leaders, conversely, tended to insist on their way or the highway. And as there were a dozen different leaders with a dozen different “their ways” (Military dictatorship, restoration of a Monarchy, Democracy; independent states, a restoration of the Tsarist Empire; etc.), they as great a job in driving away allies as the Reds did in courting them.
Of course, once the war was over, the Reds reneged on most of their promises to their allies, but since the Whites were defeated, there was no hope for them to be saved from the Reds…
Also, it is important to take into account the passive support the Germans provided the Reds. Once Lenin reached agreement with the Germans, it freed up thousands of battle experienced soldiers to serve in the Red Army. The monarchy had lost most of it’s legitimacy in the eyes of many Russians and as other have noted, the Whites didn’t offer a unifyied cause to rally around. Enough of the soldiers joined the Red Army (or were conscripted) to defeat the Whites.
But wouldn’t that have handicapped both sides equally?
No, because after the years of war, most soldiers aren’t going to be eager to fight for the very generals who commanded them during the war and who they blame for the bad conditions.