Complex sentence: By January 1943, a wide gap had opened between Army Group B and Army Group Don, and the advancing Soviet armies threatened to cut off all German forces south of the Don River, including Army Group A operating in the Caucasus.
Split into simpler sentences: By January 1943, a wide gap had opened between Army Group B and Army Group Don. The advancing Soviet armies threatened to cut off all German forces south of the Don River, including Army Group A operating in the Caucasus.