Just to help Maple's pedant quotient...
Alan Clark, that great wielder of the old actualite was the historian.
The line - or almost all of it - was first used (at least in the 20th Century) as far as can be ascertained by a Japanese officer who claimed that the Russians were 'Lions led by asses'. Source being Sir Ian Hamilton's diaries of his time when a staff officer assigned to observe the goings-on in the Russo-Japanese war.