> On a clear day, you can see the white cliffs of Dover from Calais, France. I believe at its narrowest point the English Channel is 17 miles wide. At its peak the German army in WW2 had ~10 million soldiers and a massive industrial war machine. Yet they couldn't cross the English Channel. They didn't even try.
Notably they were in a position of air inferiority the whole time, despite certain popular perceptions. So not really comparable. (Indeed if China, by contrast, is making preparations for an amphibious invasion, surely that says something)
> Look at a map of Ukraine and see where the front line is. The Dnipro River will feature strongly along much of it. That's not a coincidence. To cross even a river you need pontoon bridges to get tanks across and then trucks for supplies. Those bridges can be built quickly but the entire operation and any bridgehead you establish is incredibly vulernable to attack.
That the front in a somewhat evenly balanced war would stabilise on a natural obstacle isn't so surprising. We can't leap from there to say that such natural obstacles would make for stable defensive lines in a less balanced war.