In the days following the Allied landing at Normandy, the Germans would commit 49 U-boats to the English Channel in an attempt to cut off the Allied forces ashore. While this was less than a third of the total operational U-boats available to them, this was still a significant force. However, they would have little effect on the Allied forces in the Channel.

The German U-boat force had, over the course of 1943, suffered devastating attrition in the Atlantic. Better Allied radars, sonar, weaponry and tactics coupled with the closing of the air gap to make any sortie into the North Atlantic extremely dangerous for U-boats. As a result, the German submarine force was much smaller than it had once been. The 49 boats that were committed to the Channel represented those few that were in French ports on D-Day. While the Germans knew the Allies must come, trying to maintain constant forces in the Channel was extremely risky; the shallow waters were poorly suited to U-boats, and it was easily patrolled by Allied air and naval forces. As such, they were deployed in reaction to Allied moves. The commanders were not alerted to the Allied landings in Normandy until the afternoon of the 6th, and most did not leave their bases on the Bay of Biscay until after nightfall.

They would be going up against one of the heaviest concentrations of anti-submarine firepower ever assembled. The first part of the interlocking system of defence set up by the Allies consisted of the aircraft of No. 19 Group RAF. With 25 squadrons and a total of 350 aircraft, mostly flown by veteran ASW pilots (including this author's grandfather), it was tasked with carrying out 'Operation Cork'. This called for the sweeping of some 20,000 square miles of ocean, bounded by the Loire estuary, the south of Ireland, the English Channel Coast and the Cotentin Peninsula. The patrol routes for this were carefully laid out, such that every patch of sea within those 20,000 square miles would be swept by radar at least once every 30 minutes. Any U-boat detected would be attacked on sight. The next step was an outer barrier of ASW ships, patrolling between Brest in France and Land's End. The RN deployed four Escort Groups and the RCN two, with the escort groups being a mix of destroyers, frigates, corvettes and sloops. These groups were supported by two escort carriers, carrying fighters, to protect the hunters from any attacks by German aircraft. Further in was another barrier, this time composed of four destroyer groups, two RN and two RCN, patrolling west of the Cotentin. All ten of these escort groups were veterans, with plenty of experience in the Atlantic. Finally, the invasion convoys, and follow-ups, were protected by escorts, many of whom were also heavily experienced. To aid in locating and attacking U-boats, the escorts and hunting groups were issued with wreck charts and specialised radio-navigation aids. Altogether, the Allies had 286 ASW vessels of all types protecting the invasion. There were also heavy ASW air and naval patrols around Britain's north coast, and in the western end of the Channel, to prevent any U-boats from Norway interfering.

The 49 U-boats found it practically impossible to find their way through these heavy patrols. Eight U-boats left Brest on the 6th. They almost immediately came under heavy attack, with two being damaged and forced to turn back. The next night, two more were sunk by the same aircraft of 224 Squadron RAF, in a single flight. The next day, three more would be sunk by aircraft. On the 12th, the last boat was withdrawn, but it would be sunk by aircraft on the 18th. Those that left other bases had similar experiences. The withdrawal order of the 12th applied to all conventionally powered boats, but those with the 'schnorkel' (which allowed them to run their diesel engines while submerged) were to continue the attack, as they were more likely to avoid the air patrols. These managed to attack several of the ASW craft in the outer barriers, but didn't make it much further into the Channel. Only five boats would make it back to France successfully. The air offensive slackened in July, as the aircraft were needed elsewhere. Instead, the burden of the work fell on the ASW surface groups. The lack of heavy air patrols allowed the U-boats to get closer to the convoy routes to the beachhead, but they still had little effect. Six U-boats were sunk in the Channel over the course of July, with two more being sunk by aircraft and one by mine. Ultimately, the advancing Allies would force the Germans to withdraw from the French bases. While U-boats continued to attack Allied shipping in the Channel from their Norwegian bases, they had similarly little effect, sinking only a few craft.

They tried.

First, the U-boats couldn't sit there waiting for the invasion fleet - the time and date weren't known, and they would have patrol the expected area the invasion fleet would pass through in advance. This would have led to a lot of U-boats being sunk for no gain. Also, since the invasion wasn't where it was expected (near Calais, rather than Normandy), most such patrols would have been in the wrong place, and any lucky survivors ineffective.

Thus, the U-boats would have to respond to the invasion, rather than try to stop the invasion. This is what was tried. It was at first suspected that the Normandy invasion was a feint, with a real invasion to follow at Calais, so the initial response was slow. The only German naval attack during the day on the 6th was an E-boat attack (which sank a destroyer). About 46 U-boats were available in the general area (in the Bay of Biscay), and 36 of these were assigned to anti-invasion duty. The first U-boat attack was attempted during the night, but after 2 U-boats were sunk and 4 damaged, without reaching the invasion area, the attempt was called off.

Further attempts to attack the invasion fleet were made over the next few days, especially by U-boats equipped with snorkels. Some reached the invasion fleet and made successful attacks, but failed to inflict serious damage on the fleet (how could it have been otherwise, with so few U-boats and so large a fleet?). This damage was also inflicted after the initial landings, and after the beachhead was secure, so couldn't do more that disrupt supply and reinforcement. In the end, U-boats managed to sink 7 escorts, 3 LSTs, and 13 transports, for the loss of 20 U-boats (the first 2 sunk, and 18 snorkel-equipped U-boats). About half of the U-boats were sunk by escorts, and about half by patrol aircraft (2 by escorts and aircraft working together, and one was lost to a mine).

The E-boats also continued there attacks, sinking some more ships, in return for heavy losses of their own. The number of E-boats available was also reduced by air attacks on their bases.

A German destroyer flotilla attempted to attack, but was intercepted by Allied destroyers and driven off with 2 sunk.