After the Soviet victory in the battle of Warsaw, a communist Poland had been established. (The USSR had considered annexing Poland, but decided not to do so in light of Western opposition. Britain and France considered directly intervening to save Poland, but did not have the political or military will to do so). This put Soviet forces in Poland as "guests" of the new regime and right on the border with Germany. Additionally, the USSR exploits the Soviet-Lithuanian treaty to also put a communist government in power there, with Soviet troops also occupying that country.
The Allies worry about communism spreading to Germany and begin to create more favorable conditions for the Weimar Republic, specifically with regards to reparations and levels of military forces. This allows Germany to rearm slowly and the Germany economy to recover faster.
Within the USSR itself, the triumph over Poland causes the internal conflict over Lenin's succession to go more the way of Trotsky and world revolution. A failed communist rebellion in Italy against Mussolini, as well as communist uprisings in Germany, Finland, Latvia and Estonia throughout the twenties also help Trotsky rise to power and push out Stalin after Lenin's death.
Once in power, Trotsky begins a program of providing aid to communist groups around the world, especially in Europe and Asia. He particularly provides a great deal of money and even arms to the German communist movement.
A failed uprising by the German Communist Party in late 1928 is the trigger for the war. German forces attempt to chase communist guerrillas across the border into Poland. They clash with Soviet forces. Before diplomacy can stop it, a greater war has begun.
As far as how the war would go, I wouldn't profess to being a military expert. My guess would be that the Red Army would be large, but not that advanced, while the German forces would be better and larger than in OTL, but still not that effective, still suffering from the impact of losing WWI.
I believe that a large part of the war would be fought at a civil/guerrilla level as right-wing and left-wing forces in all the involved countries (including perhaps the USSR) would take advantage of the conflict to strike at their opponents and their government.
The next question would be what would the West do? Would Britain and France be willing to openly help Germany? Or would they only send money and arms? I think that Japan would be the most likely to actually attack the USSR, or at least to threaten to.
I think under this scenario, a quick victory could be achieved by the Soviets, overwhelming Germany; this might cause France and Britain to openly intervene of behalf of the Germans and maybe we see West Germany and East Germany 15 years earlier. IMO, though, I think such a war would actually drag on, with no quick victories. And in the worst case scenario, it would turn into a European civil war, as communists and anti-communists fight across the entire continent, perhaps aiding the rise of fascism in response or maybe even leading to an eventual communist victory.