An important question over the next few days is whether the retreating Russians can stabilize a frontline along one of the major rivers of northeastern Ukraine, most notably the Donets (or Seversky Donets) River and the Oskil River and Reservoir.
If the Russians can’t regroup and hold the line at the north-south Oskol River flowing through Kupyansk, the next north-south river of notable size in northeastern Ukraine is the smaller Aydar, at least half-way back to the Russian border from Kupyansk.
During WWII, river crossings were a sizable challenge for the Allies after D-Day, especially the Rhine. As I’ve mentioned before, the great talking rabbit novel Watership Down is a retelling of the desperate escape by British paratroopers across the Rhine in September 1944 after Operation Market Garden’s attempt to seize nine bridges turned out to be a bridge too far for British armor to reach and link up with the airborne troops.
It took almost another half a year for the Allies to get across the Rhine. On March 7, 1945, the U.S. Army unexpectedly found itself with an opportunity to race across the Rhine at Remagen south of Bonn in the minutes before the Germans could finish blowing up their Ludendorff Bridge. The 1st Army made the most of their chance in a story that’s extremely dramatic even in its Wikipedia recounting.
The 1969 movie The Bridge at Remagen, with George Segal and Ben Gazzara as the American soldiers who led the charge against across the bridge, is supposed to be decent (a respectable 6.7 rating on Wikipedia). It probably would have been even better except that it was being filmed in Czechoslovakia during the Dubcek Summer of 1968, when the Soviets invaded and movie crew had to run for the border.