I believe I've outlined this scenario before.
The Red Army was coming to save the day for the Hungarian Soviet Republic, but an unexpected White force cut them off at a crucial moment in the Ukraine. Had they just a little luck, the Red Army could have arrived in Hungary in time to save them from the Romanians. Had this happened, a direct connection could be formed between Hungary and Russia, allowing them to support each other. Hungary would overwhelm Slovakia and provide a second front during the Soviet-Polish War, which could distract the Poles just enough to allow Warsaw to be taken.