Long before winter came the chances had been diminished owing to the repeated delays in the advance that were caused by bad roads, and mud. The 'black earth' of the Ukraine could be turned into mud by ten minutes rain - stopping all movement until it dried. That was a heavy handicap in a race with time. It was, increased by a lack of railways in Russia - for bringing up supplies to our advancing troops. Another adverse factor was the way the Russians received continual reinforcements from their back areas, as they fell back. It seemed to us that as soon as one force was wiped out, the path was blocked by the arrival of a fresh force.

Quoted in "The Other Side of the Hill" - Page 184 - by Basil Henry Liddell Hart