Question: In which military branch did the prime minister when we joined the common market serve?

full_context: Document 1: Ministry of Defence (United Kingdom): The MoD has since been regarded as a leader in elaborating the post-Cold War organising concept of "defence diplomacy". As a result of the Strategic Defence and Security Review 2010, Prime Minister David Cameron signed a 50-year treaty with French President Nicolas Sarkozy that would have the two countries co-operate intensively in military matters. The UK is establishing air and naval bases in the Persian Gulf, located in the UAE and Bahrain. A presence in Oman is also being considered.
Document 2: The Sun (United Kingdom): Politically, The Sun in the early Murdoch years, remained nominally Labour. It supported the Labour Party led by Harold Wilson in the 1970 General Election, with the headline "Why It Must Be Labour" but by February 1974 it was calling for a vote for the Conservative Party led by Edward Heath while suggesting that it might support a Labour Party led by James Callaghan or Roy Jenkins. In the October election an editorial asserted: "ALL our instincts are left rather than right and we would vote for any able politician who would describe himself as a Social Democrat."
Document 3: Brexit: In a referendum on 23 June 2016, 51.9% of the participating UK electorate voted to leave the EU, out of a turnout of 72.2%. On 29 March 2017, the UK government invoked Article 50 of the Treaty on the European Union. The UK is thus due to leave the EU on 29 March 2019.
Document 4: Heath ministry: Edward Heath of the Conservative Party formed the Heath ministry and was appointed Prime Minister of the United Kingdom by Queen Elizabeth II on 19 June 1970, following the 18 June general election. Heath's ministry ended after the February 1974 general election, which produced a hung parliament, leading to the formation of a minority government by Harold Wilson of the Labour Party.
Document 5: 1975 United Kingdom European Communities membership referendum: The United Kingdom European Communities referendum of 1975, also known as the Common Market referendum and EEC membership referendum was non-binding referendum held on 5 June 1975 in the United Kingdom to gauge support for the country's continued membership of the European Communities (EC) -- often known at the time as the ``Common Market ''-- which it had entered on 1 January 1973 under the Conservative government of Edward Heath. Labour's manifesto for the October 1974 general election had promised that the people would decide`` through the ballot box'' whether to remain in the EC.
Document 6: 2016 United Kingdom European Union membership referendum: The planned referendum was included in the Queen's Speech on 27 May 2015. It was suggested at the time that Cameron was planning to hold the referendum in October 2016, but the European Union Referendum Act 2015, which authorised it, went before the House of Commons the following day, just three weeks after the election. On the bill's second reading on 9 June, members of the House of Commons voted by 544 to 53 in favour, endorsing the principle of holding a referendum, with only the Scottish National Party voting against. In contrast to the Labour Party's position prior to the 2015 general election under Miliband, acting Labour leader Harriet Harman committed her party to supporting plans for an EU referendum by 2017.To enable the referendum to take place, the European Union Referendum Act was passed by the Parliament of the United Kingdom. It extended to include and take legislative effect in Gibraltar, and received royal assent on 17 December 2015. The Act was, in turn, confirmed, enacted and implemented in Gibraltar by the European Union (Referendum) Act 2016 (Gibraltar), which was passed by the Gibraltar Parliament and entered into law upon receiving the assent of the Governor of Gibraltar on 28 January 2016.
Document 7: European Union law: Following the election of the UK Labour Party to government in 1997, the UK formally subscribed to the Agreement on Social Policy, which allowed it to be included with minor amendments as the Social Chapter of the 1997 Treaty of Amsterdam. The UK subsequently adopted the main legislation previously agreed under the Agreement on Social Policy, the 1994 Works Council Directive, which required workforce consultation in businesses, and the 1996 Parental Leave Directive. In the 10 years following the 1997 Treaty of Amsterdam and adoption of the Social Chapter the European Union has undertaken policy initiatives in various social policy areas, including labour and industry relations, equal opportunity, health and safety, public health, protection of children, the disabled and elderly, poverty, migrant workers, education, training and youth.
Document 8: Ministry of Defence (United Kingdom): Winston Churchill, on forming his government in 1940, created the office of Minister of Defence to exercise ministerial control over the Chiefs of Staff Committee and to co-ordinate defence matters. The post was held by the Prime Minister of the day until Clement Attlee's government introduced the Ministry of Defence Act of 1946. The new ministry was headed by a Minister of Defence who possessed a seat in the Cabinet. The three existing service Ministers—the Secretary of State for War, the First Lord of the Admiralty, and the Secretary of State for Air—remained in direct operational control of their respective services, but ceased to attend Cabinet.
Document 9: 1975 United Kingdom European Communities membership referendum: The United Kingdom European Communities membership referendum, also known as the Referendum on the European Community (Common Market), the Common Market referendum and EEC membership referendum took place on 5 June 1975 in the United Kingdom to gauge support for the country's continued membership of the European Communities (EC) -- often known at the time as the ``European Community ''and the`` Common Market'' which it had entered on 1 January 1973 under the Conservative government of Edward Heath under the provisions of the Referendum Act 1975. Labour's manifesto for the October 1974 general election had promised that the people would decide ``through the ballot box ''whether to remain in the EC.
Document 10: Ministry of Defence (United Kingdom): The Ministers and Chiefs of the Defence Staff are supported by a number of civilian, scientific and professional military advisors. The Permanent Under-Secretary of State for Defence (generally known as the Permanent Secretary) is the senior civil servant at the MoD. His or her role is to ensure the MoD operates effectively as a department of the government.
Document 11: Referendums in the United Kingdom: Until the latter half of the twentieth century the concept of a referendum was widely seen in British politics as ``unconstitutional ''and an`` alien device''. As of 2017, only three national referendums have ever been held across the whole of the United Kingdom: in 1975, 2011 and most recently in 2016.
Document 12: Brown ministry: Gordon Brown formed the Brown ministry after being invited by Queen Elizabeth II to begin a new government following the resignation of the previous Prime Minister of the United Kingdom, Tony Blair, on 27 June 2007. He took office as Prime Minister, a title he would hold until his resignation on 11 May 2010. In his inaugural cabinet, Brown appointed the United Kingdom's first female Home Secretary, Jacqui Smith.
Document 13: Dudley Smith: Smith served as a junior minister for the British Army and Employment during the Edward Heath government of 1970 to 1974.
Document 14: Brexit: In a referendum on 23 June 2016, 51.9% of the participating UK electorate voted to leave the EU, out of a turnout of 72.2%. On 29 March 2017, the UK government invoked Article 50 of the Treaty on the European Union. The UK is thus due to leave the EU at midnight on 30 March 2019 Central European Time (11 pm on 29 March 2019 GMT).
Document 15: Elizabeth II: In 1956, the British and French prime ministers, Sir Anthony Eden and Guy Mollet, discussed the possibility of France joining the Commonwealth. The proposal was never accepted and the following year France signed the Treaty of Rome, which established the European Economic Community, the precursor to the European Union. In November 1956, Britain and France invaded Egypt in an ultimately unsuccessful attempt to capture the Suez Canal. Lord Mountbatten claimed the Queen was opposed to the invasion, though Eden denied it. Eden resigned two months later.
ground_truth: British Army
sub_queries: who was prime minister when we joined the common market Answer: Edward Heath
Edward Heath >> military branch Answer: British Army