I've actually been interested in this topic myself, so I've pondered what the loss of Poland may mean. Firstly, obviously, the 'Polish corridor' becomes a non-issue and Germany gets a continuous land border with the fledgling USSR. But the Soviets are at the very limits of their capabilities. Taking down the Poles may free up enough reserves to secure the Baltic states, but that's scraping the bottom of the barrel for 1920. Nevertheless, the leadership of the Soviet Union was ideologically certain that their revolution would die out unless it spread east. Whatever Trotsky's reservations about Poland, it was his idea of 'Permanent Revolution' that set out the nature of Russia's revolution as not a singular event in October 1917 but instead a long, continuous process that inevitably turns international. The ideological backing for further advances into the west is there, but practicality forestalls anything for that year.
Given that, there would be many in the Entente leadership eager to intervene and crush the Red Menace once and for all. The French leadership would be keen to send foreign troops in due to their own misgivings about allowing Germany to re-arm. But get below that and you'll be unlikely to find support amongst the population. Responses would range among three seperate viewpoints, I imagine.
1. "After all we've been through I'm not going to support another pointless war."
2. "If the bourgeoisie expects us to wave our flags in support of their crushing of the world's first proletariat-controlled state, they've another thing coming."
3. "So the Russians invade Germany and they both grind each other into mince. This is a problem how?"
So, that being said, I expect such a crusade against the Russians would gain the support of eight countries, tops. Britain, France and Italy would send troops out of consideration of maintaining international stability. Finland, Czechoslovakia, Romania and Japan would join in out of a hunger for territory. And Germany's participation is a given, even if the Entente only allow it a back-seat out of fear of a re-armed Germany.
An attack in the autumn of 1920 by the professional armies would probably do quite well, driving the Russians back in the initial stages, but then the autumn sludge descends and Trotsky uses the opportunity to withdraw his troops to defensive positions and establish interior lines of communication like he did in the Civil War. Seeing how the Great War played out, he'd know the advantage of defence over attack and treat the 'crusade' like the Great War should have been treated: like an enormous castle siege. It wouldn't halt the armies but it would slow them down, and that's all he needs.
With the armies camping down for the winter you have the prospect of the first casualty lists returning, the introduction of conscription, the notice of increased rationing and the curbing of liberties for reasons of security...it'll be too much for the war-wracked peoples of Europe. Lenin has popular support because his country is the one being invaded, while the Entente is the one crushing a worker's government. In the spring of 1921 you're going to get riots, general strikes and worker's councils springing up from Belfast to Ploesti. And then it's anyone's guess what would happen next.
EDIT: Actually, I've just had a thought. With Europe returning to a state of utter carnal chaos, what does this bode for the United States? They may get an influx of persecuted workers, persecuted rich people and anyone else who thinks it's a good idea to flee while the fighting's still in the factory districts. The US would never in a million years get involved in this conflict for all kinds of sensible reasons, so making an even more pointed contrast between the peace and prosperity of America and the barbarity and depravation of Europe. What effects would this have on the roaring twenties?
