According to the "Dictionary of National Biography", "Since his chief, Lord Crewe, was in the Lords, important work fell to the under-secretary, in particular the South Africa Act 1909, which brought about the Union of South Africa." He became a member of the Privy Council in 1909. Seely was also amongst those Liberals who strongly supported Lloyd George's budgets of 1909 and 1910. Seely was defeated for Abercromby at the January 1910 General Election and returned to Parliament for Ilkeston in Derbyshire at a by-election later in 1910, holding that seat until 1922. In October 1910, he was awarded the Territorial Decoration. Seely then served as Under-Secretary of State for War from 1911 to 1912. As a yeomanry colonel, he did not support conscription, which General Henry Wilson favoured. “Ye Gods” was how Wilson greeted his appointment in his diary. Seely was already a member of the Committee of Imperial Defence. In June 1912, apparently on Churchill's suggestion, Seely was promoted to the Cabinet as Secretary of State for War, in succession to Haldane. He held the post until 1914. With Sir John French he was responsible for the invitation to General Foch to attend the Army Manoeuvres of 1912 and was active in preparing the army for war with Germany. Seely supported General Wilson when he gave evidence to the Committee of Imperial Defence (CID) in November 1912 that the presence of the British Expeditionary Force on the continent would have a decisive effect in any future war. The mobility of the proposed Expeditionary Force, and in particular the development of a Flying Corps (the origin of the modern Royal Air Force) were his special interests. According to "The Times", these developments played a significant role in the victory during World War I. In April 1913 Seely told the House of Commons that the Territorial Force could see off an invasion by 70,000 men and that the General Staff opposed conscription. Sir John French (Chief of the Imperial General Staff) obtained a partial retraction after Wilson had threatened that he and his two fellow Directors at the War Office would resign in protest at the “lie”, but Wilson felt that French's recent promotion to Field Marshal had made him reluctant to clash with Liberal Ministers. During the CID “Invasion Inquiry” (debates of 1913–14 as to whether some British regular divisions should be retained at home to defeat a potential invasion), Seely lobbied in vain for all six divisions to be sent to France in the event of war.