Complex sentence: Because few Russian roads were paved, when the rains and snow came in late October and early November, most of the main roads turned to mud and combined with longer supply lines, the German advance stalled within sight of the spires of Moscow.
Split into simpler sentences: Because few Russian roads were paved, most of the main roads turned to mud when the rains and snow came in late October and early November. These quagmires combined with longer supply lines to cause the German advance to stall within sight of the spires of Moscow.