He didn't predict a swift victory. The original plans for the invasion involved fighting for months through the low countries with an estimated cost of up to 500,000 German casualties and a start date in 1942. The only serious influence Hitler had on this plan was his instruction to maintain surprise and prevent the allies from setting up strong defensive positions; he was insistent that the attack on France happen much earlier. Hitler had suggested an attack at Sedan, but this had been swiftly dismissed and it appears that he did not pursue the matter.

When von Manstein came up with the idea of a diversionary attack in the low countries and a main, armoured thrust through the Ardennes he had a difficult job to convince enough people that it would work. With the support of Guderian, who knew the terrain well from his World War 1 experiences, he drafted and submitted a series of memoranda explaining his idea.

Von Manstein's plans were rejected by the OKH and eventually von Manstein was sent to an Eastern command to get him out of the way. It was only at this point, 2 February 1940, that Hitler was introduced to the plan by von Manstein's disgruntled staff. On 17 February Hitler held a conference and adopted the revised plan, but it remained unpopular with large numbers of senior officers who didn't understand how an entire Army Group could advance through the Ardennes.

Although I am unsure about is how long Hitler thought it would take for France to fall, what is obvious is that he was surprised by the speed of the initial advance and incapable of commanding his forces effectively. The offensive started on 10 May and the breakthrough at Sedan was achieved by 13 May, but Hitler ordered that the initial advance from Sedan to be limited to 8km. This "stopping order" was indicative of Hitler's concern that the army was overstretching itself. Although the advance did resume at pace, General Halder's diary entry of 17 May reports that Hitler was very nervous about the speed of the advance and especially the risk of an attack on the southern flank of Army Group A. Whilst this was the force's greatest vulnerability, Hitler did not understand what his front-line commanders did: the French were in no state to be able to mount such an attack.

The second stopping order came on 18 May and was more definite than the first. Hitler was increasingly paranoid about a counter-attack and was trying to seriously impede the rate of advance. His commanders, however, were doing everything they would to feign ignorance of his concerns and maintain momentum (Rommel's 7^th Panzer Division made such a habit of this that it earned the nickname Ghost Division as his headquarters often didn't know where it was).

The German vanguard reached the Channel on 21 May and from here Hitler's malign influence was unavoidable. As the channel ports were cleared, and the BEF moved to Dunkirk, there was a small window of opportunity to completely encircle the remaining Allied forces. On 23 May von Rundstedt ordered a halt to the advance on Dunkirk - a decision which Hitler ultimately supported. Whilst the army re-grouped, Goering convinced Hitler that the Luftwaffe could destroy the Allies at Dunkirk from the air. Although this was utterly fanciful, Hitler supported it and the BEF was able to evacuate in great numbers.

So Hitler completely underestimated how quickly France would fall, although he could take credit for seeing the potential in the revised plan. He could be described as being ahead of his generals by siezing the initiative, but he quickly fell behind them when battle was joined. Although he was not alone in his paranoia about an Allied counter-attack, his direct interference with his front-line commanders showed his poor grasp of command. This interference was directly contrary to German doctrine, which emphasised momentum and initiative, and prevented a killer blow being delivered to the BEF. Hitler's behaviour would prove to be disastrous on the Eastern Front.