Even to this day, a contested river crossing is among the most difficult tasks a military force can receive. In WW2, the obstacle posed by crossing major rivers like the Rhine and Dnieper required enormous operations, of the same scale as those needed for major amphibious landings

A key component in a successful crossing in both WW2 and today was/is the ability to effectively suppress enemy firepower to enable units to make the crossing and expand a bridgehead, often using things like airborne/heliborne forces, air power, or long range guided artillery

However, most of these options would not have been available to a WW1 army. Enemy long range guns could be positioned where they could fire upon any bridgehead at the limit of their range, while being effectively immune to counter battery fire from the aggressor's guns that are now stuck back on the other side of the river. And WW1 armies would not have the level of tactical airpower or airborne force movements that modern armies have

Were there any instances of WW1 armies making a contested crossing of a major river, particularly in the West and after 1914? Do we have any information about how the Entente forces planned to force a crossing of the Rhine, had Germany continued to resist?