On March 15, 1918, Field Marshal Sir Douglas Haig received a momentous piece of news. His wife had borne him a son. He was 57 and had almost lost hope of there being an heir to the ancient title of Laird of Bemersyde. His biographer Duff Cooper recounts how Haig embraced the RAMC colonel who had been in attendance and, to his astonishment, “kissed me like a Frenchman”.

Less welcome news struck the field marshal after his return to France, where he was commanding almost two million Commonwealth soldiers of the British Expeditionary Force. Few British commanders in the field have carried such protracted responsibility as Haig and none survived in so senior a command having never enjoyed the confidence of the Prime Minister. Lloyd George would have sacked him on taking over in December 1916 if he felt his Coalition colleagues, parliamentary and public opinion would have accepted it.

Their difference hinged as much on policy as on personalities. Haig never swerved from the view that Germany could only be beaten on the Western Front, Lloyd George favoured a holding position there in hope that an alternative — less costly in lives — might emerge in the Balkans. In the event, the decisive battle was launched by Germany and it paved the way to Allied victory.

As Haig dressed on the morning of March 21, he was informed that the anticipated German offensive had begun with a massive onslaught 43 miles wide, stretching southwards from Arras to La Fère. The weight of the attack fell on General Sir Hubert Gough’s 5th Army, on ground recently taken over from the French Army under Petain. Its aim was to split the Allied front, cut off the BEF from the Channel ports and — in conjunction with intensification of the submarine campaign against merchant ships bringing essential supplies — force Britain out of the war.

Yet desperation had turned the German C-in-C, Field Marshal Hindenburg, and his chief of staff, Ludendorff, away from their shrewd defensive strategy on the Western Front. The vast cost in men and crucial resources of the March 1918 offensive was forced on them by the Kaiser’s approval of unrestricted submarine sinkings of neutral shipping, together with the madcap scheme to create a diversion in the event of this provoking the United States into declaring war.

In January 1917 Germany had offered Mexico financial support and lands previously lost to the USA, through an alliance “to make war together, make peace together”. But the telegram from Foreign Minister Zimmermann instructing the German ambassador in Washington to relay this proposal to Mexico City was intercepted by British Naval Intelligence and passed to the US State Department. On April 2, 1917, President Woodrow Wilson asked Congress to “accept the status of a belligerent thrust upon it”. When the German high command recognised that this meant hundreds of thousands of US troops arriving “over here”, their hopes hung on a successful offensive before enough of them arrived to influence the outcome.

Haig’s generalship, in readiness for the offensive he knew Ludendorff must launch, could hardly be faulted. He positioned his reserves in the north, where the front was 25 miles from the sea at the nearest point, and, anticipating the main German attack in the south, ordered Gough’s 5th Army to give ground but keep his front intact, while striving to maintain contact with the British 3rd Army on his left and the French 6th Army on his right.

Gough was to be made Lloyd George’s scapegoat for the British casualties sustained and the ground lost, but his withdrawal was in exact accord with instructions. He handled his divisions like a flexible chain: when it broke he cobbled it together to grant Haig time to bring the French to his aid over the ground they had until recently held.

On March 28 Ludendorff delivered a new attack with fresh divisions in the north of the BEF front at Arras, bringing him little more than crippling casualties. In his sop to public opinion, Lloyd George insisted that Gough be replaced by General Sir Henry Rawlinson and the 4th Army staff, but by then the worst was over in the south. The BEF had fought six gruelling defensive battles between March 21 and April 5, retreating almost to the gates of Amiens.

Ludendorff then swung north against Ypres and Hazebrouck, where the BEF fought eight more costly battles. The bill for March and April was 40,000 dead, 180,000 wounded and 93,000 missing from every part of Britain, Ireland and the Commonwealth, but the endurance of the survivors had thwarted the breakthrough on which Ludendorff depended.

Foresight, resolve and persistence were rewarded. Ludendorff conceded the BEF was too much for him and turned on the French. Haig began preparations for his ultimately decisive offensive in August in concert with Foch, co-ordinating the Allied effort on the Western Front.