In September 1916, Horne was created a Knight Commander of the Order of the Bath. After the successful capture of Flers, he was promoted to temporary General and succeeded Sir Charles Monro (after a brief hiatus between 7 August and 29 September when the command was held by Haig's first choice Richard Haking, who was then blocked from receiving the promotion) as commander of the First Army. On 1 January 1917, he was promoted to the substantive rank of Lieutenant-General "for distinguished service in the field". His first trial occurred in April 1917, when his troops were sent on a diversionary attack on the fearsome Vimy Ridge, which rose hundreds of feet over the surrounding landscape. French Army commander Robert Nivelle was critical of Horne's plan; Nivelle was the one found incompetent and, after one month of relative failure (and a mutiny), Nivelle was sacked and replaced with Philippe Pétain.