"in an iron cage"

Marshal Ney

Meanwhile the Bourbon Government vainly issued orders for the Emperor's arrest and dispatched increasing numbers of troops to intercept his journey.  Marshal Ney promised Louis XVIII the he would return to Paris with Napoleon "in an iron cage," but when his forces met the Emperor near Auxerre on March 14 the old attraction again proved too strong and the men once more destered en masse, followed by their commander.  In Paris some wit posted a large notice in the Palace Vendome: "From Napoleon to Louis XVIII.  My good brother -- there is no need to send any more troops -- I have enough."

~ D. Chandler, The Campaigns of Napoleon, page 1012