During the invasion of Normandy, I've read about multiple instances where the allied attack was stalled or bottlenecked in part due to the Germans intentionally flooding certain areas.
What I don't have are the details - the books I've ready so far say 'the Germans flooded X' and leave it at that. My question is, when and how did they do this? Was it done quickly in response to the invasion or was it done months or years beforehand as part of preparing the coastal defenses? And how did they get the water there - was there an irrigation effort to divert rivers or what?
And finally, was this a new defensive tactic? I don't recall reading much about similar intentional flooding in other campaigns.
I'll leave others to answer with regard to the Normandy campaign, but the use of water defences in warfare was certainly not new. They had been used by the Dutch in an attempt to delay the German invasion of 1940 and, most famously, deliberate flooding had also been a key tactic of the nascent Dutch Republic against Spain during the 80 Years War, or Dutch Revolt, in the late 16th and early 17th centuries. Water defences also played a key role for the Dutch in fighting off French invasion in 1672 and again during the Seven Years' War, in 1757. It was admittedly a truly desperate measure, for the inundation of salt water ruined fertile land and breaches in the dikes required a massive repair and drainage effort once hostilities were concluded.
Much of the northern Netherlands were, of course, geographically ideal for this sort of warfare, being low-lying, flat and reclaimed from the sea. Probably the best-known and most successful instance involving the breaching of water defences and the intentional flooding of land to deny it to an enemy came in 1574, when a powerful Spanish army was besieging the strategically important city of Leiden. By intentionally breaching the dikes that surrounded the city, the Prince of Orange was able to force the besiegers into retreat and simultaneously use the so-called 'Sea Beggar' fleet to sail inland into the newly flooded areas and relieve the city. This was an important victory, as it played a significant part in enabling the Dutch rebels to hold out until widespread mutinies in the Spanish army, caused by Spain's inability to pay its troops, gave the Dutch a two-year respite which they used to construct a strong ring of new fortresses.
The difficulties caused to any invader by the use of such water defences has been important in Dutch history in itself; the mere threat of cutting the dikes was a significant deterrent.