Selling weapons to your enemies is probably not the best idea

But the French finally seem to be learning it: in the wake of Russia's invasion of Ukraine, they have halted their delivery of warships to Russia:

France has said conditions are "not right" for delivery of the first of two Mistral navy assault ships to Russia.
President Francois Hollande's office blamed Moscow's recent actions in Ukraine.
France had until now resisted pressure to halt the delivery, saying it had to respect an existing contract.
Russia's Deputy Defence Minister Yury Borisov said the French decision would not hold back Moscow's plans to reform its armed forces.
"Although of course it is unpleasant and adds to certain tensions in relations with our French partners, the cancelling of this contract will not be a tragedy for our modernisation," he said, quoted by Itar-Tass news agency.
Ukrainian Foreign Minister Pavlo Klimkin tweeted his thanks to the French leadership for its "responsible decision", which he said was "important for restoring peace in Europe".
The Vladivostok, the first of the two helicopter carriers, was expected to have been delivered to Russia by late October.
The second, the Sevastopol, was to have been sent next year, although no mention of it was made in Mr Hollande's statement.
As the crisis has escalated in eastern Ukraine and as Russia's direct military role there has become more blatant, so the pressure on the French government to halt its sale of two advanced assault ships to Russia has grown ever stronger.
The US and a number of other countries have long made their feelings plain. But the deal weathered tensions with Moscow over Syria, and the Russian crew of the first vessel which is already undergoing sea trials has travelled to France to begin training.
This was the most significant Western arms sale to Russia and its postponement - the exact terms of the suspension of the deal are not clear - marks a very visible rebuff to Moscow on the eve of Nato's Wales Summit.
The Mistral assault ships can carry up to 16 heavy helicopters, land troops and armoured vehicles. Their delivery would have resulted in a marked improvement in Russia's amphibious capability.

In my book Rising Sun, Falling Skies: The Disastrous Java Sea Campaign of World War II, there are several instances of the Japanese using weapons sold to them by the British against the British. The Japanese battleship Kongo was completed as a battlecruiser at a Vickers shipyard Britain, the last Japanese capital ship built overseas. She was later used in the hunt for the British battleship Prince of Wales and the battlecruiser Repulse in the first days of the Pacific War. During the Battle of the Java Sea, the British cruiser Exeter was disabled by an 8-inch shell, believed to have come from the Japanese cruiser Haguro, that exploded in one of her boilers. When they were attempting emergency repairs to the boiler room in Soerabaja, the Royal Navy engineers found the shell's base plate. It read "Made in Britain."