A phrase often attributed to Talleyrand (1754-1838). Supposedly, after the battle of Leipzig, in October 1813 (or at some other occasion - the Hundred Days are often mentioned in alternative versions of the story), Talleyrand said to Napoleon: "C'est la commencement de la fin".
The thought does not appear to be original with Talleyrand, though. In William Shakespeare's A Midsummer Night's Dream, Act 5, Scene 1, one finds the words "That is the true beginning of our end".
A commonly quoted variation on this theme is found in Winston Churchill's words (in a speech, November 11, 1942): "This is not the end. It is not even the beginning of the end. But it is perhaps the end of the beginning."