During 1927 Desmond Morton was heavily involved in a lengthy operation which contributed to the conviction in January 1928 of Wilfred Macartney and a German Communist, Georg Hansen, for offences under the Official Secrets Act. Macartney, who had worked for MI6 under Compton Mackenzie during the war, was as much an inept petty criminal and confidence trickster as a spy. Indeed, Mackenzie afterwards referred to him as being in jail for "comic opera espionage". But the evidence against him, especially as detailed by Morton (who appeared before the court as "Peter Hamilton"), linked him through Hansen to Arcos and Soviet intelligence. Morton's interest in the Macartney case was heightened by his position, in addition to his Production role, as head of another new SIS department, Section VI, formed during 1926-27 to gather intelligence on the economic preparations for war of potential enemies including Germany and the Soviet Union.

Like Section V, Section VI was closely concerned with Communist activities at home as well as overseas, and Morton was also involved in a collaborative operation run by SIS and MI5 from 1925 investigating William Norman Ewer, the foreign editor of the Daily Herald, who was suspected of running a Communist espionage network. While SIS kept watch on the group abroad, MI5 ran postal and telephone surveillance at home. In the spring of 1929 Sinclair told the Foreign Office that the operation conclusively proved that Ewer's group "were conducting Secret service activities on behalf of, and with money supplied by, the Soviet Government and the Communist Party of Great Britain". In 1928 Albert Allen (an ex-policeman whose real name was Arthur Francis Lakey) told MI5 that two Special Branch men had been working for Ewer since 1922, which led to the arrest in April 1929 of Inspector Ginhoven and Sergeant Jane, along with an ex-policeman, Walter E. Dale. Among the evidence seized was a diary kept by Dale which revealed that between 1922 and 1927 "unremitting surveillance" had been "maintained by Dale and his friends upon the premises and personnel of S.I.S and of the Code and Cipher School", that "laborious efforts were made to identify and trace to their homes officers and members of the secretarial staff of both organisations, "and that the move of both offices ... to joint premises at Broadway Buildings under a common style was accurately observed and recorded". One result of this was the removal from the publicly available Post Office London Directory of the anodyne entry "Government Communications Ltd" among the tenants of Broadway Buildings which had appeared in the 1928 and 1929 edition listings.

Allen also reported to MI5 that in 1923 a secretary at SIS's Melbury Road headquarters, Mrs Moon, had been targeted by Ewer's agent, Rose Edwardes, who approached the woman "representing herself to be a member of the American Intelligence Service". She offered Moon £5 a week to work for her, "and a good bonus for any useful pieces of information she might be able to obtain". But the SIS secretary "apparently got nervous and eventually refused to take up the work", although over "a number of conversations" Edwardes had apparently been "able to find out a good deal of what went on at Melbury Road so far as it was known to Mrs Moon". Moon had at the time reported the approach to the authorities but, reflected Jasper Harker of MI5, she had "suppressed a good deal" and had not been "anxious to give such information as would put us on the track of the mysterious lady from the American Secret Service who was supposed to have approached her".

In late March 1927, an acquaintance of Macartney's, a young insurance broker named George Monkland, made contact with Admiral "Blinker" Hall, saying that he had "come across something curious which he thought would be of interest to the British Government": a lunch was arranged on 29 March with Hall and the former Deputy Chief of SIS, Freddie Browning, who still maintained an active interest in Intelligence matters. Monkland handed over a document - described by Desmond Morton in his statement as "a most intricate and exhaustive questionnaire on the Air Force of Great Britain"; Admiral Hall, on Browning's recommendation, passed it not to MI5, but to Sinclair, who gave it to Morton. The latter, according to his own account, immediately recognised the document to be "the work of an expert" and considered that it "could only have had origin in the Government Offices of a foreign power", probably Soviet Russia; an assessment that was confirmed by the Air Ministry, though they thought it was "so complete that [it] must have been compiled by the united efforts of several experts".

Morton lost no time in making contact with Monkland according to the procedures suggested by Browning, using "emerald" as a code word and introducing himself as "Peter Hamilton". Morton said he adopted an assumed name because he "could not discover any mutual acquaintance" with Monkland, providing confirmation that it was his practice only to use an alias when dealing with contacts outside the Establishment "circle", or (as in the case of Makgill) when any meetings or correspondence were likely to extend to "outsiders". Morton and Monkland met for the first time the next day, 30 March 1927. Their encounter was the first of many contacts in the next eight months, during which period Monkland acted as an intermediary between "Peter Hamilton" (who also used the codename "Sunfish") and Macartney, in an attempt to get the latter to incriminate himself, and in the process lead SIS to further Soviet agents and operations.

A remarkable communist, with whom I made friends at this time, was Wilfred Macartney, the author of a book called Walls Have Mouths, which had made a strong impression on me. It was an account of his experiences in prison, where he had served a long sentence for a clumsy attempt at espionage. White-haired and rubicund, with the manner of a hard-drinking journalist, he had a vitality which his years in prison appeared to have done nothing to diminish. He was the most conspicuous example that I have ever come across of those who combine left-wing opinions with an appetite for high-living. I think of him in an expensive hotel suite, which he had the air of having annexed, drinking champagne and surrounded by pretty girls, being still unwilling to deny the possibility that the defendants in the Moscow trials had really been guilty of the charges brought against them. He was not a man whom one could altogether admire but one would have needed to be a greater puritan than I was not to enjoy his company.

The first commander of the British Battalion was Wilfred Macartney. Although not a communist, he had been sent to prison in Great Britain for disclosing military secrets to the Russians. His account of his experience, The Walls Have Mouths, became a huge success." He was, at first, highly regarded by both the men and the commissars. Springhall wrote, "McCartney [sic] has taken over as commandant of the Battalion. He is making a big hit with the lads & I think will do the job very well.... He has always acted with decisiveness & showed clear thinking and good military leadership."

By January 6, however, there were hints of trouble. Peter Kerrigan believed that disciplinary problems and a bad cold had considerably affected Macartney's spirits. Kerrigan asked Pollitt if he would send Macartney a letter singing his praises. It would also help, he said, if the "DW [Daily Worker] ... put [him] across big."" Kerrigan still was hopeful that Macartney would prove to be an effective leader. "He really is very capable indeed and in my opinion well respected by the men." Two weeks later Macartney's fellow writer and second-in-command, Tom Wintringham, reported that Macartney was "doing great work." But he echoed Kerrigan's concerns to Harry Pollitt that his commander labored "under a heavy load of discouragement, and you would be wise to try to get some cheering messages to him." Only now was the seriousness of the situation conceded by Wintringham. "He is not now talking about resigning, as he was a week ago, but he is still showing temperament." In addition, Wintringham believed his superior was too soft-hearted in administering discipline."

On the same day that Wintringham wrote, Kerrigan told Pollitt more harshly, "Now McCartney [sic] is a problem and a worry." Kerrigan still believed he was the only one who could command the battalion, but the Scot's patience was wearing thin. "My impression about [him] is that he is far too irritable or querulous and I feel this has an effect on his ability to inspire the men with confidence in himself."" George Aitken, the steady political commissar who had been brought out from England, added with quiet understatement, "Mac is a rather difficult man to handle."" But the most damning indictment that Kerrigan lay before the head of the British Communist party was that Macartney had become "very critical of the Party." On February 1st Kerrigan told Pollitt that "the Zero hour is coming quickly," probably referring to the approach of battle but perhaps also to the crisis in the battalion leadership.

The early Britishers had passed through the Thaelmann and other battalions, the French at least. I know that there were Slavs as well, because I came across and got very friendly with a little Bulgarian representative who was a base commissar for the Bulgarians. So there were Eastern Europeans and Polish there. Later, among the British, there were Cypriots and others. Among the Americans were Cubans. The battalion at this time was about 600 [men], with my group, No 1 Company, and the other volunteers that were trickling in. In January a big thing happened: a packing case arrived with new Russian rifles. You fired these rifles with the bayonet on and they made a very big difference. There was also the old Maxim, an old-fashioned heavy machine gun, but rather deadly when it was being used, especially if at not too long a range.

The command structure in the British Battalion at this point was that each company had a company commander with NCOs below them. Above them all was the battalion commander (Wilfred McCartney), the deputy commander (Tom Wintringham), the second-in-command and the battalion political commissar. I was the base commissar. It was at about this time that (George) Nathan was taken away to divisional headquarters. He became Chief of Staff at division and he was killed at Brunete. He was a very brave man. He worked by example as well as being disciplined.

I think I should mention here what happened in connection with McCartney, who had to go back to England. He didn't ask to go back but he had to go back. I understand that he had done 9 years of his 12-year sentence for espionage. He wrote a book afterwards called Walls Have Mouths, which was about his experiences in prison. I don't know if he was guilty or not but he was found guilty. Anyway, he was going back and I visited him in his room before he went back to have a talk with him about the situation with the battalion and so on. It was the intention that he would come back. This was about mid-January but he had a big, heavy revolver and I had a rather small Belgian revolver, and he said: "Look Peter, how about you giving me your revolver. I am going through France I don't want to lump this thing about". I said all right. He asked to show me how to operate it. I took the revolver in my hand but I can't say for sure whether or not I touched the safety catch, or whether it was off or not, or whether I touched the trigger, but suddenly there was a shot and I had hit him in the arm with a bullet from the small Belgian revolver. We rushed him to hospital, got him an anti-tetanus injection and he was patched up and off he went.

Why did he go back? I am certain it was not to get him out of the command position although he was, how shall I put it, a British military officer type. He was not terribly popular in the battalion but I think he was respected for his ability. He was a capable military officer. He had a rather arrogant style. I was not given the job of getting rid of him. I have nothing to hide about it. It was an accident.'

McCartney was unpopular. A former British Army officer recently released from prison for spying for the Soviet Union, he was not a Communist. He frequently lost his temper and he held hierarchical views about his status that were out of place in a people's army, however disciplined. When news came through from London that he had to return to England because his ticket of leave after prison parole had expired, no tears were shed. Then an extraordinary accident occurred. On 6 February, Kerrigan entertained McCartney to a farewell dinner in Albacete during which he persuaded him to leave behind his big Mauser pistol in exchange for Kerrigan's Belgian .22. The Mauser was loaded and accidently Kerrigan touched the trigger, shooting McCartney in the arm and instantly rendering him unfit for combat. By such an improbable accident - for it was an accident, despite suspicions to the contrary - did Tom Wintringham become Commander of the British Battalion on the eve of its first battle.

Tom Wintringham was second-in-command and Peter Kerrigan was political commander of the brigade. We were going to the front and Wilfred McCartney didn't want to go back. He said he was going with the fellows to the front. Peter Kerrigan and the rest of us thought he shouldn't, and it so happens that he shot him in the arm to make him go back to hospital. That was the only way to get him back because we didn't want to give him a bad name.

This was a tremendous shock! Nobody liked McCartney with his 1914 ideas but to find your battalion commander is not there when you are about to go into battle and a kind of instructor is in command! Well, it's a very demoralising thing, particularly in these mysterious circumstances. All sorts of rumours started to go round: "McCartney's deserted!" "McCartney's committed suicide!" "Pete Kerrigan's shot McCartney!" This turned out to be true.