Archibald Sinclair , 1st Viscount Thurso Archibald Henry Macdonald Sinclair , 1st Viscount Thurso KT , CMG , PC ( 22 October 1890 – 15 June 1970 ) , known as Sir Archibald Sinclair , Bt between 1912 and 1952 , and often as Archie Sinclair , was a British politician and leader of the Liberal Party . Background and education . Sinclair was the son of a Scottish father and an American mother . He was the great - great - grandson of Sir John Sinclair , 1st Baronet . In 1912 he succeeded his grandfather Sir John Sinclair , 3rd Baronet as fourth Baronet , of Ulbster . Educated at Eton College and the Royal Military Academy Sandhurst , he was commissioned into the Life Guards in 1910 . Military career . Sinclair served on the Western Front during the First World War and rose to the rank of Major in the Guards Machine Gun Regiment . He served as second - in - command to Winston Churchill when Churchill commanded the 6th Battalion of the Royal Scots Fusiliers in the Ploegsteert Wood sector of the Western Front in 1916 , after Churchill had resigned as First Lord of the Admiralty . They formed a lasting friendship that would become a significant political alliance in later decades . From 1919 to 1921 he served as Personal Military Secretary to Churchill when he returned to the Cabinet as Secretary of State for War , then accompanied him to the Colonial Office as Private Secretary . Political career . In 1922 Sinclair entered the House of Commons as a Liberal Member of Parliament ( MP ) for Caithness and Sutherland , supporting David Lloyd George and defeating the incumbent Liberal supporter of H . H . Asquith . His constituency was the largest , in terms of area , in the United Kingdom . He rose through the Liberal ranks as the party shrank in Parliament , becoming Chief Whip by 1930 . In 1931 the Liberal Party joined the National Government of Ramsay MacDonald , with Sinclair appointed Secretary of State for Scotland . He was sworn of the Privy Council at the same time . The following year he , together with other Liberal ministers , resigned from the government in protest at the Ottawa Conference introducing a series of tariff agreements . Sinclair and the Liberal leader , Sir Herbert Samuel , were thus the last Liberal politicians to sit in the Cabinet until 2010 . In the 1935 general election , Samuel lost his seat . Sinclair became the party ' s leader at the head of only twenty MPs . With the party now clearly marginalised as the third party on the fringe , with few distinct domestic policies , with a parliamentary party that was primarily a collection of individuals elected as much for themselves as for their party , and with the separate Liberal Nationals offering competition amongst Liberal - inclined voters , Sinclair fought to make the Liberals once more a relevant force in British politics , taking up the issues of opposition to the continental dictatorships and working closely with Winston Churchill who was a backbencher at that time and generally shunned by his Conservative Party . When Churchill formed an all - party coalition government in 1940 , Sinclair became Secretary of State for Air . However he did not sit in the small War Cabinet , though he was invited to attend meetings discussing any political matter . As Secretary for Air , he played a leading role in planning the fire - bombing and destruction of Dresden . He remained a minister until May 1945 when the coalition ended . In the 1945 general election , he narrowly lost his seat . His margin of defeat is one of the tightest on record - he came third , even though the victor had only 61 votes more than he . There was speculation that he might return to the Commons and the leadership , as the Conservative victor in his seat had promised to only serve in parliament until the end of war with Japan , a pledge he kept modifying to serving just one more year , every year . Sinclair awaited the imminent by - election , which never materialised . At the 1950 general election Sinclair again stood for his old seat and moved to second place , but in yet another close election , he remained 269 votes away from victory . In 1952 , the year of his first stroke , he accepted elevation to the House of Lords as Viscount Thurso , of Ulbster in the County of Caithness . He was expected to take up the leadership of the Liberal group in the House of Lords , but a much more serious stroke in 1959 left him largely bedridden and in a state of precarious health until his death in 1970 . Family . Sinclair married Marigold ( 1897 - 1975 ) , daughter of Lieutenant - Colonel James Stewart Forbes , in 1918 . They had four children : the Hon . Catherine ( 1919 - 2007 ) , the Hon . Elizabeth ( 1921 - 1994 ) , Robin ( 1922 – 1995 ) , and the Hon . Angus ( born 1925 ) . Sinclair was one of the largest landowners in the United Kingdom , owning an estate of about 100 , 000 acres ( 400 km² ) in Caithness . He was handsome and charming and regarded as a daredevil , but in private life was rather shy , reserved and antisocial , with a slight speech impediment . In the 1990s , his grandson , John Thurso , entered politics and has sat since 2001 as the Liberal Democrat MP for his grandfather ' s seat , now called Caithness , Sutherland and Easter Ross . Sinclair ' s granddaughter , Veronica Linklater , Baroness Linklater of Butterstone ( daughter of the Hon . Elizabeth Sinclair ) , is also a Liberal Democrat politician .