The Germans could not immediately seize the French fleet in 1940 for several reasons.

One key reason was that the French fleet was not easily accessible to them. Much of the French fleet was in port in North and West Africa. The battleship Richelieu was being moved to Dakar; the Force de Raid, with the large cruisers Strasbourg and Dunkerque and the old battleships Dunkerque and Provence, was at Mers-el-Kebir. The aircraft carrier Bearn was in the West Indies. Other ships were in British ports. The battleship Lorraine was at Alexandria, while Paris and Courbet were in Plymouth and Portsmouth. Seizing the ships in the French ports in North Africa might have been possible, with cooperation from the French. Seizing ships in British ports was all but impossible.

Another major reason was that the armistice signed between the Germans and French did not allow the Germans to seize the French navy. Article 8 of the armistice, as submitted on the 22nd June, stated that:

The German Government solemnly declares to the French Government that it has no intention of using during the war for its own purposes the French Fleet stationed in ports under German supervision, other than the units necessary for coastal patrol and mine sweeping. It further declares solemnly and formally that it has no intention of making claims in respect of the French Fleet after the conclusion of peace.

Any attempt to breach the terms of the armistice would have catalysed support for Free French forces. Doing it so soon after signing it would be seen as a major breach of trust, and might well have led to the French in North Africa continuing to fight with the Allies. Omitting such a clause, or inserting a clause which would allow the Germans to take the French Navy, would have had similar effects. This was why the Germans made no attempt to capture the French fleet until after Operation Torch in 1942. The perceived cooperation of the Vichy regime with the Allied landings was seen by the Germans as a breach of the armistice, and provided the pretext for the seizure.

Seizing the French Fleet would also have required the French, and British, to do nothing to prevent the seizure. The French admiralty had no interest in seeing their fleet under German control. They were very willing to scuttle their ships, rather than to hand them over to the Germans. They took actions to prepare for this course of action. On the 24th June, Admiral Darlan transmitted a message to every ship in the French Navy. This emphasised that the French Navy was not to be allowed to fall into German hands:

1. The demobilised warships will stay French, under the French Flag, with reduced French crews, remaining in French metropolitan or colonial ports.
2. Secret preparations for auto-sabotage are to be made in order that an enemy or foreigner seizing a vessel by force shall not be able to make use of the same.
3. Should the Armistice Commission charged with interpreting the text, come to a different decision from that in para 1, and as soon as action on any such decision is attempted, warships are without further orders to be despatched to the United States, or alternatively scuttled, provided that no other action is possible to escape the enemy. Under no circumstances are they to fall into enemy hands.

This course of action was followed in November 1942, after the Germans attempted to seize the French fleet at Toulon; 77 ships were scuttled, including every modern battleship and cruiser. Similarly, the British had no incentive to allow the Germans to take possession of the French Fleet. The Royal Navy was ordered to take steps to prevent French ships being moved to anywhere where the Germans might be able to get to them. These could even involve sinking the ships, as they did at Mers-el-Kebir on the 3rd July 1940.

Finally, there were also logistical issues. The Kriegsmarine was a comparatively small force. It did not have the men to man a greatly expanded force. Even if it stripped the ships it did have to man the French ships, and recruited more men, they would need time to train with the new weapons and equipment. There was little commonality in terms of ammunition and spare parts between the French and German ships. This would increase training time, and increase logistical difficulties. The Germans would have been relying on French stocks of ammunition and parts, unless they diverted production capacity to producing them - but these parts could not be used elsewhere in the German fleet.

Had the Germans managed to capture the French fleet, as a whole, man them, and make them ready to fight, it would not have made Operation Sealion possible. The French had five battleships, all older, plus two more modern large cruisers. None had guns larger than 13.5in. Richelieu was under construction, but nearly complete and could have been brought into action. They also had an aircraft carrier, the Bearn, but she was small, old, slow, and had no modern aircraft. This added to two German large cruisers with 11in guns. In comparison, the RN could call on eleven battleships and three battlecruisers, all of which had 15in or 16in guns. It also had six carriers. While three of these were small, old and slow (and for two of them, in the Far East), the remaining three individually outclassed Bearn considerably. The disparity was similarly large in terms of cruisers and destroyers.