That's part of it, but the descendants of Napoleon's armies - the best specimens of young French manhood - were obliterated in WWI, taking that martial spirit right out of the gene pool.
The Russians lost more youth ("the best specimens of young [Russian] manhood") than the French did (even as a percentage of the population in WW2). That didn't make the Russia a nation of cowards. I think it has more to do with the liberal welfare society and socialism.