Nothing that I can think of, he led reform efforts, including development of naval aviation, tanks, and the switch in fuel from coal to oil, a massive engineering task, also depending on securing Mesopotamia's oil rights, bought circa 1907 through the secret service using the Royal Burmah Oil Company as a front company. Unless you actually meant WWII in which he was eventually prime minister. Edit: Perhaps you mean his delay in implimenting the newly developed tank as a standard fighting fixture? He saw them as a misappropriation of funds. The tank was deployed too early and in too small numbers, much to Churchill's annoyance. He wanted a fleet of tanks used to surprise the Germans under cover of smoke, and to open a large section of the trenches by crushing barbed wire and creating a breakthrough sector. Or perhaps you mean his influence in the mistake of the Gallipoli landings? In 1915, Churchill was one of the political and military engineers of the disastrous Gallipoli landings on the Dardanelles during World War I. Churchill took much of the blame for the fiasco, and when Prime Minister Asquith formed an all-party coalition government, the Conservatives demanded Churchill's demotion as the price for entry.

The failure of the Gallipoli campaign is in part his responsibility.