The attack on Mers-el-Kébir (Battle of Mers-el-Kébir) on 3 July 1940, during the Second World War, was a British naval attack on neutral French Navy ships at the naval base at Mers El Kébir, near Oran, on the coast of French Algeria.
The attack was the main part of Operation Catapult, a British plan to neutralise or destroy neutral French ships to prevent them from falling into German hands after the Allied defeat in the Battle of France. The British bombardment of the base killed 1,297 French servicemen, sank a battleship and damaged five other ships, for a British loss of five aircraft shot down and two crewmen killed. The attack by air and sea was conducted by the Royal Navy, after France had signed armistices with Germany and Italy, coming into effect on 25 June. 
For Churchill and his War Cabinet, the risk of the French war ships falling into enemy hands was too paramount and, in the early hours of July 2, Sommerville received the following signal for French Admiral Gensoul;
"It is impossible for us, your comrades up to now, to allow your fine ships to fall into the power of the German or Italian enemy. We are determined to fight on until the end, and if we win, as we think we shall, we shall never forget that France was our Ally, that our interests are the same as hers, and that our common enemy is Germany. Should we conquer, we solemnly declare that we shall restore the greatness and territory of France. For this purpose, we must make sure that the best ships of the French Navy are not used against us by the common foe. In these circumstances, His Majesty's Government have instructed me to demand that the French Fleet now at Mers el Kebir and Oran shall act in accordance with one of the following alternatives;
(a) sail with us and continue the fight until victory against the Germans and Italians.
(b) Sail with reduced crews under our control to a British port. The reduced crews would be repatriated at the earliest moment.
If either of these courses is adopted by you, we will restore your ships to France at the conclusion of the war or pay full compensation, if they are damaged meanwhile.
(c) Alternatively, if you feel bound to stipulate that your ships should not be used against the Germans or Italians unless these break the Armistice, then sail them with us with reduced crews, to some French port in the West Indies - Martinique for instance - where they can be demilitarised to our satisfaction, or perhaps be entrusted to the United States and remain safe until the end of the war, the crews being repatriated.
If you refuse these fair offers, I must, with profound regret, require you to sink your ships within 6 hours.
Finally, failing the above, I have the orders from His Majesty's Government to use whatever force may be necessary to prevent your ships from falling into German or Italian hands."
Admiral Gensoul replied in writing that in no circumstances would his ships fall into German or Italian hands and, ominously, that force would be met with force. The distress felt by the British Admiral and his senior staff was evident in the exchanges of signals with London. The ships they were about to fire on were crewed by men who had been their allies just 10 days earlier. Understanding the difficulties, Churchill instructed the Admiralty to send the following message to Somerville;
"You are charged with one of the most disagreeable and difficult tasks that a British Admiral has ever been faced with, but we have complete confidence in you and rely on you to carry it out relentlessly."
The final signal, dispatched at 6.26 pm read,
"French ships must comply with our terms, sink themselves or be sunk by you before dark,"... but the action had already started at 5.45 pm.
Operation Catapult commenced on July 3, 1940. Early in the day, all French warships in British territorial waters were boarded and impounded by the Royal Navy (code named Operation Grasp). This amounted to two battleships, four cruisers, eight destroyers, some submarines, numerous support vessels and smaller craft, which had fled when the collapse of France seemed inevitable. This part of the operation went relatively smoothly, however, resistance did occur on the French submarine, Surcouf, resulting in the deaths of one French sailor and a Royal Naval Rating, plus several others injured.
Later in the day "Force H", with flagship, HMS Hood, arrived off the coast near Mers-El-Kebir. A three-point ultimatum was sent to Admiral Gensoul, the French commander, giving him the following options;
Bring out your ships and join the Royal Navy.
Take the fleet to a British port with a reduced crew from where they would be repatriated.
Sail the fleet to a French, West Indian or an American port and decommission the fleet there.
Gensoul decided not to act on this, preferring to open a dialogue with the British chief negotiator, Captain Holland of HMS Ark Royal. However, Somerville soon became aware of Gensoul’s vacillation and a fourth option was added to the earlier ultimatum "scuttle your ships where they lie."
A little after 1pm, the British dispatched Swordfish planes, from the carrier Ark Royal, to mine the harbour entrance. This action angered Gensoul, who felt the British had acted in bad faith. However, despite the heightened tension, outwardly all remained calm until 4:46 pm. Somerville received a communiqué from the Admiralty, which considerably raised the stakes. It stated that Somerville had "to settle matters quickly" as French reinforcements were on their way.
Somerville wasted no time. At 5:15 pm, he signalled to the Battle cruiser, Dunkerque, that if his proposals were not met by 5:30 pm, he would have to destroy their ships. The French failed to respond. Captain Holland’s negotiations had failed. Action stations were sounded then the first salvo from the Hood’s fifteen inch guns smashed into the side of the French battleship Bretange, causing fatal damage. She sank with the loss of 977 crew members.
For fifteen minutes, H Force’s guns ranged down on the French fleet in the harbour, causing death and destruction. The French had been badly mauled. Apart from the sinking of the Bretagne, the Dunkerque was crippled with 200 dead and many injured, the destroyer Provence had run aground and Mogador was badly damaged.
Gensoul then signalled a cease-fire, to which Somerville replied "unless I see your ships sinking I shall open fire again." As a precaution, Somerville moved H Force out of the range of the French guns. He assumed that his mines would stop any breakout by the remaining French, along with aircraft from the Ark Royal, took up the pursuit. However, Somerville soon brought the chase to a halt, since the absence of these vessels left the remaining blockading ships too vulnerable to attack. The Strasbourg later arrived at Toulon, where it was once more under Vichy French control. Despite this setback, the French squadron had effectively been neutralised but at a very high cost in human lives.
Further along the coast at Alexandria, a second British battle force had assembled to confront a substantial part of the remaining French navy in the southern Mediterranean. This time, the British commander at Alexandria, Admiral Cunningham, was able to open a successful dialogue with his friend and French counterpart, Admiral Godfroy. Despite orders from Churchill for results to be achieved by nightfall, he held the negotiations over till the next day, July 4 and a settlement was reached. Godfroy’s eleven ships were immobilised in Alexandria harbour with the draining of their oil supplies and the confiscation of their breech blocks by the French consulate at the port.
The Vichy French Government was, understandably, angry and dismayed by the turn of events at Mers-El-Kebir and other ports. The British had killed 1,200 French sailors, who had been, just two weeks earlier, their close allies. In addition, they had seized, immobilised or sunk a large part of the French navy.
In some French quarters, support for the British cause waned and Petain broke off diplomatic relations with Britain. Two days later, the Vichy French captured three British merchant ships in retaliation. Further skirmishes between the former Allies occurred, including the bombing of Gibraltar by the French and the torpedoing of the French battleship, Richelieu, at Dakar, by the British.
The reaction in the UK to Churchill's determination and resolution in seeing through this grotesquely difficult problem, was largely positive. It proved that his recent appointment to the position of prime minister, was a good move. When the action was announced to Parliament, there was cheering from both sides of the house. The action had further benefits for Britain in the USA, as confirmed by Roosevelt later when he told Churchill that his decisive action against the French Navy had convinced him that Britain still had the will to fight, even if she was alone.
Ultimately, the action at Mers-El-Kebir was a tragedy. However, the French navy no longer posed a threat to the UK's conduct of the war at a time when she was most vulnerable.
Summary of Action
Allied Forces: Sea - Force H - HMS Hood (Battleship) Vice- Admiral, Sir James Somerville. Flag, HMS Resolution (Battleship), HMS Valiant (Battleship), HMS Ark Royal (Carrier), HMS Arethusa (Cruiser), HMS Enterprise (Cruiser), HMS Faulkner (Destroyer), HMS Foxhound (Destroyer), HMS Fearless (Destroyer), HMS Forester (Destroyer), HMS Foresight (Destroyer), HMS Escort (Destroyer), HMS Kepple (Destroyer), HMS Active (Destroyer), HMS Wrestler (Destroyer), HMS Vidette (Destroyer), HMS Vortigern (Destroyer), HMS Ark Royals aircraft consisted of: 12 Skuas (800 Squadron), 12 Skuas (803 Squadron), 12 Swordfish (810 Squadron), 9 Swordfish (818 Squadron), 9 Swordfish (820 Squadron).
Vichy French Forces: Sea - Admiral Marcel-Bruno Gensoul, Strasbourg (Battle cruiser), Dunkerque (Battle cruiser), 2nd Battleship Division. Rear- Admiral Bouxin, Provence (Battleship), Bretagne (Battleship), Mogador (Destroyer), Volta (Destroyer), Tigre (Destroyer), Lynx (Destroyer), Kersaint (Destroyer), Le Terrible (Destroyer).
Located to the east of Mers-El-Kebir at the port of Oran were the following French forces. Light Destroyers: 10, Submarines: 6 ( 3 Operational), Assorted smaller ships 13. Land - French Shore Batteries; Fort Santoni: 3x194mm Guns, Gambetta Battery: 4x120mm Guns, Espagnole Battery: 2x75mm Guns, Canastel Battery: 3x240mm Guns.
Royal Marines manned around 1/3 of all guns on Capital Ships as well as other roles such as manning gun control positions, buglers, butchers and and most likely Royal Marines airmen flying in the Air Squadrons involved.
In conjunction with Catapult Operations Grasp was conducted on the night of the 3rd July in which Royal Marines would certainly have made up many of the boarding parties, and certainly made up about half of the party that boarded the Submarine Surcouf.
Related Royal Marines 'Dits':
References/ Further Reading:
 Wikipedia - Attack on Mers-el-Kébir
 Combined Operations - Operation Catapult - Mers-el-Kebir, North Africa
 HMS Hood.org.uk