During World War II official use of the Red Ensign was authorized by the RCAF in 1943 (Group 6?) and by the Canadian Army in 1944.
http://www.fotw.net/flags/ca-1921.html
This history I think may be definitive (forgive the length). It would seem to me that the Legion is being a-historical in thinking the Ensign was in common use at the front in WW I.
THE ARMS WITH GREEN LEAVES
http://fraser.cc/FlagsCan/Nation/Ensigns.html
"It is often said that Canadian nationalism came home in the baggage of the soldiers from the World War I. The soldiers of the Canadian Expeditionary Force had fought World War I under the Union Flag, which, as members of the British Empire, was their flag as much as anyone else's. However, they desired to be recognized as Canadians, and this was provided, at least in part, by their maple-leaf-covered badges.
The increased Canadian consciousness that came home with the soldiers in 1919 caused the pendulum to swing from the Union Flag back towards the Canadian Red Ensign. The shift was not strong enough that it would have been possible to persuade the country to adopt the ensign as the national flag, as was evidenced by the fuss in the country and parliament when a government committee was appointed in 1925 to report on the adoption of a national flag. The support for the Union Flag forced Prime Minister Mackenzie King to disband the committee. Nevertheless, the Canadian Red Ensign was coming back into its own.
The Canadian Red Ensign that emerged shortly after the war, was not the cluttered and aberrant ensigns of the past. The new badge was the shield of the recently granted arms of Canada. The previous situation where each province had arms, but the Dominion had not, was inappropriate, especially in the light of the increased feelings of Canadian nationalism. On March 26, 1919, Prime Minister, Sir Robert Borden appointed a committee to advise on a granting of arms from King George V.54 The arms they proposed were, with a most curious and significant change, the arms that George V granted on November 21, 1921.
Based on the committee report, on April 30, 1921 a Canadian government order-in-council requested a shield which had as its base, "argent three maple leaves slipped vert" or three green maple leaves upon a silver field. It was the colour of the maple leaves on the shield that was at issue, for the maple leaf held by the lion in the crest was red, and the mantling was red and white.55 The mantling established Canada's national colours as red and white. The red maple leaf in the crest, consistent with the national colours, was in striking contrast to the colour of the leaves on the shield. When the proclamation of arms arrived, it read "Argent three maple leaves conjoined on one stem proper." The "proper" indicated that the colour of the leaves on the shield was to be that of natural maple leaves, which included not only the requested green, but also yellow, or red. This subtlety was not to bother anyone for many years; for now, green leaves it was.
One other detail of interest for the ensign badge was third quarter of the shield: the Irish harp. This had been merely specified as "Azure A harp or stringed argent" which meant a golden harp with silver strings on a blue field. At the time the arms were granted, this was interpreted as being the same representation of a harp as appears upon the royal arms (and royal standard): a harp bearing the naked torso of a winged maiden. This, like the colour of the maple leaves, was to change in a later version of the arms, and so also on the badge of the ensigns.
On April 26, 1922, by order-in-council, the government authorized the shield of the recently granted arms to be used as the badge on both the Canadian Red and Blue Ensigns.56 This created the second official form of these ensigns, and now this satisfactory badge displaced all of the previous aberrant ones.
By now the Canadian Blue Ensign and its twin the Canadian Blue Jack had four different functions. As an ensign, it was worn at the stern of all governmental vessels other than warships, and it replaced the Canadian Red Ensign at the stern of merchantmen if the Captain and some of the crew were officers in the Royal Canadian Naval Reserve. As a jack it appeared at the bow of all governmental ships, whether warships or not. However, on warships the jack had the same shape as the ensigns: twice as long as it was wide. It also appeared at the bow of other government vessels, but there, the Canadian Blue Jack was nearly square. Thus, essentially the same flag, distinguishable only by its shape, flew at both the bow and the stern of non-belligerent governmental vessels. For example, RCMP cruisers in the 1930s flew a Canadian Blue Ensign at the stern and a (square) Canadian Blue Jack at the bow.57
On January 26, 1924, two years after the new badge was authorized, the official use of the Canadian Red Ensign was extended by another order-in-council. Now the flag was authorized for use on all Canadian buildings outside Canada. Although it now represented the Dominion in London, Geneva, and, shortly afterwards, in Washington, Paris and Tokyo, the Union Flag still flew over the Parliament Buildings at home.58 Although sentiments of Canadian nationalism were rising in the west, and they had always been high in Québec, Ontario remained fervently imperialistic. As an editorial in the Toronto Mail and Empire of June 5, 1925, put it:
'... all the rules the ordinary flag owner need pay attention to are few and simple. For him is just one flag that can properly be flown; that is the Union Jack.' 59
In a somewhat more descriptive than prescriptive mood, a 1926 "manual of Canadian citizenship" produced by the National Council of Education billed the Canadian Red Ensign as the "National Emblem of Canada." School children of the day were assured that in addition to the use of the Canadian ensign on ships and on Canadian buildings abroad, it
'is used at home by many of our citizens within the boundaries of the Dominion itself, as a symbol of our national freedom and independence within the Empire.' 60
Up until 1934, the use of the Canadian Red Ensign by merchant ships was based on the acquiescence in 1892 of the British Admiralty to the Canadian request. With formal independence gained in 1931 through the Statute of Westminster, the Canadian government moved to establish its own shipping regulations, and, concomitantly, authority over its merchant flag. The 1934 Canada Shipping Act read:
'The red ensign usually worn by merchant ships with the shield of the coat of arms of Canada in the fly is hereby declared to be the proper national colours for all ships registered in Canada and all ships and boats which would be registered in Canada if they were required to be registered at all.' 61
While questions of flag usage can lie unsettled for many years during peacetime, a war forces them to be addressed. In World War I, the Union Flag failed to distinguish Canadian soldiers as anything but a part of the great British effort. The Canadian independence and self-assurance that followed that first great conflict would not permit a similar merging of identities a second time. To distinguish the Canadian combatants, the War Committee of the Cabinet had the Battle Flag created (approved December 7, 1939). Designed by Colonel A. Fortescue Duguid, Director of the Historical Section of the General Staff of the National Defence Headquarters in Ottawa, it had a white field, the Union Flag in the canton, three red maple leaves on one stem in the centre, and three golden fleurs-de-lis on a blue roundel in the upper fly.
In the early years of World War II, patriotic illustrations representing the services would show three flags: this battle flag of the army, the light blue ensign of the RCAF (approved in July 5, 1940), and the white ensign of the RCN. Although the Union Flag appeared in the canton of each flag, it rarely appeared on its own in such illustrations, as had been the pattern in the first war in spite of the fact that the King's Rules and Regulations (Canada) 1939, familiarly known as "K R Can" stated categorically that the flag of Canada "was the Union Jack."62 When the country as a whole was to be illustrated, the Canadian Red Ensign was invariably used.
The battle flag went overseas with troops, but it was a montage that sought to please many, and consequently pleased few. An editorial in The Maple Leaf, an armed forces newspaper published in London, noted that there was "Overwhelming opposition to the Canadian flag proposed by Col. Duguid ... [as] is shown in letters which have deluged The Maple Leaf office." (December 10, 1945) The flag fell into disuse. Meanwhile, the Canadian Red Ensign was gaining ground.
In 1943 August, during one of the periodic meetings at which the western allies' strategy was decided, Prime Minister Mackenzie King was host in Québec to British Prime Minister Winston Churchill and American President Franklin D. Roosevelt. This occasion demanded the hoisting of the flags of the three participant countries; the use of the Canadian Red Ensign for Canada was likely the first time in forty years that it had been used officially by the government to represent the country upon Canadian territory. The arrangements, presumably made by Britain, were at first not satisfactory. King was annoyed when he saw "the Canadian flag beneath the Union Jack, the day that Churchill arrived."63 After speaking to Churchill about the slight, he ordered all the flags to be flown at the same height with the Canadian Red Ensign in the central position of honour.
The ensign's stock continued to rise as its use was extended to both the air force and the army. On November 10, 1943, a routine order stated:
'The Canadian Red Ensign with a shield of the Coat-of-Arms of Canada in the fly is to be flown in addition to the R.C.A.F. Ensign, at all units of the R.C.A.F. serving with the forces of other nations.'
Shortly thereafter, on January 22, 1944, a comparable routine order extended the use of the Canadian Red Ensign to the army. That Mackenzie King was a convert was clear from his recommendation to the cabinet on April 28, 1944:
'that Canada take the Canadian Ensign and accept it at once as her national flag; not wait to design a special flag. Later a Committee could be appointed to consider new designs.'
However, nothing more was done officially until the war was over a year and a half later. 64
Thus, for the latter third of the war, not only the Canadian forces knew that they were fighting under the Canadian Red Ensign, but publications such as the Star Weekly, a weekend newspaper supplement, made the public well aware of it.
As the previous conflict had done, World War II enhanced national pride and confidence; many of those soldiers who went overseas thinking of themselves as British subjects came back as Canadians. So it was that the House of Commons was informed on October 1, 1945, that:
'The Red Ensign is being flown from the Tower of the Houses of Parliament under authority of Order-in-Council, P.C. 5888, passed September 5, 1945. ... The Order-in-Council contained these provisions:
That until such time as action is taken by parliament for the formal adoption of a national flag, it is desirable to authorize the flying of the Canadian Red Ensign on federal government buildings within as well as without Canada, and to remove any doubt as to the propriety of flying the Canadian Red Ensign wherever place or occasion may make it desirable to fly a distinctive Canadian flag.
Nothing herein shall be deemed to alter in any way the provisions now in force with respect to the flying of the Blue Ensign with the Shield of the coat of Arms of Canada in the fly on Canadian naval vessels and other government vessels, nor with respect to the flying of the Canadian Red Ensign on Canadian Merchant vessels.'
Finally, after having flown the Canadian Red Ensign informally for about three-quarters of a century, Canadians had received official sanction for their actions. The order-in-council stopped short of declaring it the national flag of Canada, instead gave it a provisional status "until such time as action is taken by parliament for the formal adoption of a national flag". This was a point that was often ignored over the next twenty years by ardent supporters of the flag. 65
Use of the Ensign at Vimy would be utterly wrong, it would seem.
Mark
Ottawa