Why did the British open fire on the French in the Attack on Mers-el-Kébir during WWII? From what I understand the British were fully aware that the French people were 100% forced to surrender to the Germans. So why would the British attack a French ship with only French soldiers on board?
The British attacked the French ships to ensure that they would not be seized by the Germans. If captured the French ships would be a significant boost to the German navy, and with the Royal Navy being one of Britain's main defences the British couldn't allow the German navy to be able to challenge them (as the Kriegsmarine was much smaller than the Royal Navy).
The most powerful French flotilla was at Mers-el-Kebir, consisting of 4 battleships and 6 destroyers, so the British decided to send a task force to neutralise the threat of them being captured. The commander of the task force sent to Mers-el-Kebir, Force H, was ordered to deliver an ultimatum to the French demanding the fleet do one of the following:
Sail with the British and continue to fight against the Germans.
Sail with reduced crews to a British port.
Sail with reduced crews to a French port in the West Indies.
Scuttle the ships within 6 hours.
Failing the above, the British were authorised to use whatever force necessary to prevent the Germans from capturing the ships.
The French rejected the terms and Churchill ordered Force H to resolve the impasse quickly. The commander of Force H radioed the French commander to warn them if they didn't accept one of the proposals within 15 minutes they would have to sink their ships. The French wouldn't budge so the British ships opened fire and disabled the fleet, preventing the ships from being of any use to the Germans.
TL;DR: The British attacked the French ships to ensure that the Germans would not be able to capture them and use them against the British.
Sources:
http://www.militaryhistoryonline.com/wwii/articles/merselkebir.aspx
French Battleships 1922–1956, Jordan, John and Robert Dumas (2009)
The French govenremnt was not 100% forced to surrender to the Germans. Non of the other countries gave up, Belgium, Norway, Netherlands, Poles all keeped fighing. The French were in fact far more capable of continuing to fight because the had a huge Empire with signifiacant amounts of troupes and a big Navy.
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