Every country is apt to make blunders in War, as recent events have so sadly reminded us, but it is important to distinguish between genuine 'blunders' and 'missed opportunities', which are only apparent with hindsight.
Arguably the biggest blunder made by the Germans in World War Two was under-estimating their opponents. A country whose leader dismissed the British leadership striving for peace in 1938 as 'Little Worms' and the whole of the Slavic nations as 'Untermenschen' was always likely to over-reach itself sooner or later. Hitler's casual declaration of war on the United States after Japan attacked Pearl Harbor was simply more of the same.
Some have argued that 'Dunkirk' was a German blunder. This is highly questionable, though it may have been a missed opportunity. The Germans had fooled the Allies with their strategy in May 1940. The Allies expected Germany to do what it had done in 1914 and attack through Belgium, which was still neutral and would not admit Allied troops. With the impregnable Maginot line to the south along the Franco-German border, Belgium was the weak spot and the French First Army, which included the 250,000 British Expeditionary Force, was deployed on the Franco-Belgian border ready to advance when Germany attacked. The attack, when it came, was a ruse to draw the Allies forward whilst the main German thrust was delivered to their south, through the Ardennes forest. This area of the border had been left very weakly defended as it was agreed to be impenetrable to mechanized armored forces.
Led by Heinz Guderian, German Panzers moved through the Ardennes undetected and swept into France, brushing aside the weak defenses at Sedan, crossing the river Meuse on May 12th and 13th. With air support they moved swiftly through lightly defended territory behind the main French forces, causing panic in Paris. A nervous Hitler ordered a halt from May 15th -17th. Guderian's thrust had been more successful more quickly than anyone had thought possible. He was counter-attacked by the French from the south on May 18th but beat this off and was able to continue his advance towards the English Channel. This was reached at Abbevile on May 19th, trapping the Allied forces which had been lured into Belgium in a huge pocket. A British attack at Arras to the south of the pocket alarmed the Germans, but was only a momentary success.
From the German point of view, the risk now was that Guderian's long line of advance had outrun his infantry support, leaving him vulnerable to counter-attacks from south and north which could cut his supply lines. Panzers were dependent on vast supplies of petrol to keep going. In the confusion of battle no one had the complete picture and Hitler ordered Guderian to sit tight for several days, from May 24th to 26th, to allow infantry to catch up. Then he was able to resume his advance along the French coast, capturing the ports from which any Allied soldiers would have to be rescued and constricting the pocket in which the British Expeditionary Force was now trapped. There was still a significant British garrison in Calais. Most of the BEF still held a front against advancing German troops coming through Belgium. It was only on May 26th that the Germans realized the potential of Dunkirk. At the same time the British commander, Lord Gort, had decided to ignore his orders to try to break out to the south and had ordered a retreat to Dunkirk, seeing it as the last port in Allied hands which could be held long enough for an evacuation to be attempted.
The British Admiralty began Operation Dynamo, the Dunkirk evacuation, on the night of May 26th and it ran until June 4th. The expectation was that perhaps 40,000 to 50,000 men might be saved. German aircraft bombed and strafed the soldiers on the beaches. The shallow sea made approach by big ships possible only alongside a mole reaching precariously out to sea. These became sitting targets for German bombers and were at risk from mines. Most men had to wade neck deep into the water in line and wait for small boats to pick them up. From there some were put aboard naval vessels waiting offshore and others were ferried direct to England. Others waited their turn on the beaches, attacked from the air repeatedly. Nine hundred vessels took part in the evacuation, including 39 destroyers, 36 minesweepers, 77 trawlers and 26 yachts! The perimeter of the Dunkirk pocket was held by a rearguard, but the loss of heavy equipment made it only a matter of time until the Germans finally snuffed it out.
Neither the Germans nor the British expected that 334,000 troops could be evacuated from Dunkirk. Had Guderian advanced sooner, it does not follow that Dunkirk would have fallen to his tanks and that the entire British Expeditionary Force would have been captured. To his south Guderian still had the bulk of France's undefeated army. The British had a resourceful and courageous leader in Lord Gort and were in no mood to throw in the towel. The Royal Air Force had stood up well to the Luftwaffe and remained a potent force able to give some air cover. Although tactically and strategically outmatched by Germany's 'blitzkrieg', and the attack through the Ardennes, the BEF was a professional army not raw recruits and had given a good account of itself. It is likely that the BEF would have fortified Dunkirk whatever Guderian had done and used it as a departure point, but they might have done it sooner. The German failure to advance upon Dunkirk sooner was only a 'blunder' if they were capable of clairvoyance! They identified it as a place of key importance at the same time as the British did and then made every effort to capture it and prevent the escape of its defenders. So much for Dunkirk.
The Battle of Stalingrad really was a blunder, and of epic proportions. Hitler must take the blame for it, dooming an entire Army Group to destruction. After launching his invasion of Russia in June 1941 he had shown an unhelpful tendency to interfere with his Generals from long distance. In July 1941 he could not refrain from hindering Center Group's advance on Moscow by ordering Guderian's Panzers south towards Kiev and 1st Panzer Group north to take prisoners. This delayed the advance and left the Germans short of Moscow and exposed to the brutal winter cold without winter clothing. Worse by far was what followed.
The German advance resumed in the south of Russia in summer of 1942, aiming to secure crucial oil supplies by capturing the oilfields of the Caucasus. General von Paulus, commanding over a million men of the 6th and 4th Panzer Armies, was engaged in this when instructed to capture the city of Stalingrad, straddling the river Volga. Although it was a communication and manufacturing center, the city was not of any great strategic significance and did not pose a major threat to von Paulus' operations. Hitler had a visceral hatred of Communism and of Stalin and was determined to inflict on him the humiliation of capturing the city which bore his name. Accordingly, in September 1942 von Paulus' forces moved upon Stalingrad. The Russians, led by Zhukov, prepared to defend it.
As the fighting developed the entire city was reduced to rubble. The Germans captured most of it but were unable to clear the Russians from the eastern shore of the Volga. Brutal hand to hand fighting, street by rubble choked street, ensued with huge losses on both sides. On November 19th a massive Russian counter-attack was launched, which entirely encircled Stalingrad, trapping the Germans inside it. Von Paulus asked for permission to break out to join other Russian forces to his west. This would have been entirely possible but Hitler forbade any retreat, ordering his army to stand fast and fight to the last man. Short of supplies and suffering horribly in the icy cold of the Russian winter, the Germans held on in the rubble of Stalingrad as the Russian encirclement tightened. Repeated requests by von Paulus to Hitler were met with the same answer; no retreat, no pull back, stand and fight to the death.
Obedient to his master, Von Paulus did hold on until January 31st 1943. His fellow General in Stalingrad, Schreck, held out until February 2nd. Ninety one thousand German soldiers, all who remained alive from the vast force which had besieged the city in September 1942, were eventually surrendered to the Russians. Of these, barely a handful survived captivity to return to Germany after the War. An entire Army Group had been sacrificed pointlessly to satisfy the vanity and obsessive hatred of one man. The German Army was never able to make good these losses and from 1943 onwards the Russians went onto the offensive. German moral was undermined and Russian moral boosted. It was a blunder of monumental proportions.