Complex sentence: The Soviets promised to invade Romania and link up with Kun and were on verge of doing so, but military reversals suffered by the Red Army in Ukraine stopped the invasion of Romania before it began.
Split into simpler sentences: The Soviets promised to invade Romania and link up with Kun, and were on verge of doing so. However, military reversals suffered by the Red Army in Ukraine stopped the invasion of Romania before it began.