Archibald Henry Macdonald Sinclair, 1st Viscount ThursoKTCMGPC (22 October1890 – 15 June1970), known as Sir Archibald Sinclair, 4th Baronet from 1912 until 1952, and often as Archie Sinclair, was a Scottish politician and leader of the BritishLiberal Party.
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 worked 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 firebombing 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 59 votes more than him.
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 was 269 votes away from victory. In 1952 he accepted elevation to the House of Lords as Viscount Thurso. He was expected to take up the leadership of the Liberal group in the House of Lords, but a series of strokes in the mid-fifties left him in a state of precarious health until his death in 1970.
Sinclair married Marigold Forbes in 1918. They had four children: Catherine (born 1919), Elizabeth (born 1921), Robin (born 1922), and 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.