Is there a famous quote saying something to the effect of ‘everything we've learned about x we learned through blood’?
How about Lessons not learned in blood are soon forgotten. In googling this, I've found it attributed to both Abraham Lincoln and Carl von Clausewitz. I haven't been able to confirm either, but I do know that the actor Gerard Butler said it in the movie Law Abiding Citizen.
I found this by searching online, but it might be the closest one to your quote:
I learned in Army Special Forces that all lessons are learned in blood.
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