20 March, 20 days before the attack (Z minus 20), beginning with destructive fire aimed at isolated German batteries, leaving the 'nests' of grouped batteries to be neutralised later by HE and gas shelling. From Z minus 10 the CB groups switched to more vigorous destruction of all known enemy batteries, telephone exchanges and observation posts. The huge weight of artillery, including 'some of the most accurate counter-battery fire of the war', together with the dash of the attacking troops ensured the success of the attack when it was launched on 9 April. After the initial success at Vimy, the Arras Offensive broke down into costly trench warfare. First Army was engaged in several actions though April and May 1917. 137th (Deptford) Hvy Bty transferred to 67th HAG on 14 May, but returned to 15th on 4 July as First Army improved its positions around Oppy Wood and Souchez. Later, while much of the BEF was engaged in the Third Ypres Offensive, I Corps and Canadian Corps fought the Battle of Hill 70 north of Lens. For this operation 15th HAG supported the Canadians with CB fire beginning in mid-July, although the attack was postponed until 15 August, by which time only 63 of the 102 identified German batteries were still active. The assault was successful, but several counter-attacks had to be beaten back, and fighting went on until 25 August. Passchendaele The Canadian Corps was due to be moved north to join Second Army in the Ypres offensive and on 6 October the battery was transferred to 2nd Canadian HAG. The Corps went into the line between 14 and 21 October for the Second Battle of Passchendaele. Second Army's autumn attacks at the Menin Road, Polygon Wood and Broodseinde (see above) had been highly successful because of the weight of artillery brought to bear on German positions. But as the offensive continued into the winter the tables were turned: British batteries were clearly observable from the Passchendaele Ridge and were subjected to CB fire, while their own guns sank into the mud and became difficult to aim and fire. The Canadians attacked on 26 October with ground conditions that became increasingly bad as rain fell; they made no more than , the other attacking troops even less. The battery transferred to 42nd HAG with the neighbouring Fifth Army on 30 October for the second phase of the battle, but