The “defining ideological conflict of the 1930s”  was the Spanish Civil War which took place between July 1936 and March 1939. The conflict, which began as a right-wing military revolt in July 1936 under the leadership of General Francisco Franco, threatened to become the Sarajevo of the Second World War. During the three years that followed, Franco’s Spanish Nationalists, supported by Italy and Germany engaged in a civil war with the Loyalists, supported by the Soviet Union [although on a much smaller scale]. At the end of the three years, Franco was victorious. Yet, by the time the Second World War began in September 1939, the Labour Party was unified behind a policy of armed confrontation with Nazism and Fascism, something it was not in 1936. 
The Spanish Civil War was, perhaps, the most controversial foreign policy issue in 20th Century British politics. Initially, the question of whether arms should be sent to help the Loyalists deeply divided the Labour Party.  When the Spanish Civil War broke out, the Party’s leader, Clement Attlee, was in Russia, and most of the other political leaders were outside the country as well. The only leaders who were able to make a decision were all from the right-wing, trade union, faction of the Party.  They decided to recommend a policy of non-intervention to the National Government, as long as it was “applied immediately, are [sic] loyally observed by all parties, and their execution is effectively co-ordinated and supervised….,” a resolution that was approved at the 1936 Annual Party Conference held in Edinburgh. 
That conference was the most contentious of the entire decade. It was in many ways because, by October, it became obvious that the policy of non-intervention agreed to [by some of the Party leaders] in the Summer was a farce. There was growing evidence that the Germans and the Italians were not abiding by the Non-Intervention Treaty signed by them, as well as Great Britain, France, and the Soviet Union in late August.
The National Executive Committee [the chief policy-making body of the Labour Party] put forward a resolution the first day that supported non-intervention. Arthur Greenwood, the Deputy Leader of the P.L.P., apologetically defended the policy, saying that it was “a very, very bad second best.”  Despite a great deal of idealistic opposition, the resolution was approved by a vote of 1,836,000 votes to 519,000 on the first day.  This vote, however, did not settle the issue, much to the chagrin of the Party leadership. Two days later [October 7, 1936], the Conference heard from two Spanish fraternal delegates: Señor de Asna and Señora de Palencia. Señor de Asna spoke first:
….We [the Loyalists] are fighting with sticks and knives against tanks and aircraft and guns, and it revolts the conscience of the world that that should be true. We must have arms. Help us to buy them somewhere in the world … Is it too much to ask that in this fight against the Fascisms of the world, the democracies shall allow us to buy the arms we need to fight this battle, which we are certain we can win for democracy and for the peace of the world. 
In his speech, he cited seven instances of German or Italian violation of the Non-Intervention Pact since early September.  Next, Señora de Palencia, [who spoke flawless English as she was half-Scottish], gave an emotional speech:
….But let me tell you, if you wish this atrocious war to end soon come and help us as you have been asked, whenever you can. Think of the difference in the price of lives of two months to one year. Think of the precious gift that is being wasted—- of the lives of our youth. Do not tarry. Now you know the truth. Now you know what the situation is. Come and help us. Come and help us. Scotsmen, ye ken noo !! [emphasis mine]. 
The two speeches moved the conference. It immediately directed Attlee and Greenwood, the Parliamentary leaders, to go immediately to London and confer with the National Government over the problem.  As more and more evidence of Italian and German aggression became available in October, the Labour Party reconsidered its position regarding non-intervention. On October 29, 1936, Attlee stated this shift in a major Parliamentary debate over Spain, criticizing the non-intervention policy as
… something that is quite abnormal. One reads in the newspapers statements as if we [the Labour Party] were clamouring for intervention, and that we were asking for interference in the affairs of another country. Quite the contrary. What we are asking is that there should be a restoration of normal international relations [to the legitimate Spanish Government: the Loyalists]… 
He then stated the general Labour Party position on foreign policy and attacked the Government:
….We want to stand by democracy. But what do we see? We see a steady retreat by democracies, a surrender of liberty and democracy to every threat. That is not the way of safety for this country. 
He advocated collective security, and said that “In the last few years we have seen democracies waning because they have not stood together.”  He also denounced the Government’s foreign policy as one that had led to “… a world in anarchy.” 
Two months later he attacked the entire basis on which the National Government’s foriegn policy had been based:
You have states [i.e. Germany and Italy] which entirely disregard obligations. They tear up treaties that are made, and they carry on making a new treaty while breaking another. We [the Labour Party] feel that it is very little use to have signatures unless signatures are to be honoured. 
He also criticized the Foreign Secretary [Anthony Eden], saying:
There was a remarkable cartoon the other day of the right hon. Gentleman as an autograph hunter going with a book before these dictators and getting signatures. It was a bitter cartoon and contained a good deal of truth. 
By March 1937, there were 110,000 “foreign volunteers” inside Spain, mainly Italian and German.  Mussolini openly boasted about his “volunteers,” yet the British Government attempted to maintain the fiction of non-intervention. Again they did not want to contemplate decisive action for fear of war.
To this Attlee replied that he did not believe in “throwing sops to Dictators.”  He ended his speech with a stark warning, saying: “We [the Labour Party] believe in democracy, but if democracy is to survive it must be prepared to stand up to the dictators.” 
 Kenneth Millen-Penn, “From Liberal to Socialist Internationalism: Konni Zilliacus and the League of Nations, 1894-1939, Ph.D. Dissertation, (Binghamton, NY: Binghamton University, 1993), p. 545.
 John F. Naylor, Labour’s International Policy: Labour in the 1930s, (London: Weidenfeld and Nicolson, 1969), p. 138.
 Kenneth Harris, Attlee, (London: Weidenfeld and Nicolson, 1982), p. 127.
 Naylor, p. 143.
 Ibid., p. 144, quoting Labour Party Annual Conference Report, 1936, p. 29.
 Ibid., p. 162.
 At Labour Party Conferences, delegates from affiliated organizations [Trade Unions, constituency parties, auxiliary groups,e tc.] cast votes equal to their total national memberships.
 At this time, there was an Embargo on the sale of arms to either side of the conflict. Franco and his Nationalists still got their arms, albeit covertly from Italy and Germany. The Loyalists did not receive as much arms, and thus, were at a distinct disadvantage.
 Naylor, pp. 163-64, quoting Labour Party Annual Conference Report, 1936, p. 213.
 Ibid., quoting Labour Party Annual Conference Report, 1936, p. 215.
 Ibid., pp. 164-65.
 House of Commons Debates, vol. 316, [316 H. C. Debs.], 29 October 1936, col. 136.
 Ibid., col. 140.
 319 H.C. Debs., 19 January 1937, col. 109.
 Charles Loch Mowat, Britain Between the Wars, 1918-1940, (Chicago: The University of Chicago Press, 1955), p. 573.
 319 H.C. Debs., 19 January 1937, col. 111.
 Ibid., col. 115.