Protracted action at Verdun throughout 1916, combined with the bloodletting at the Somme, brought the exhausted French army to the brink of collapse. Futile attempts using frontal assault came at a high price for both the British and the French and led to the widespread French Army Mutinies, after the failure of the costly Nivelle Offensive of April-May 1917. The concurrent British Battle of Arras was more limited in scope, and more successful, although ultimately of little strategic value. A smaller part of the Arras offensive, the capture of Vimy Ridge by the Canadian Corps, became highly significant to that country: the idea that Canada's national identity was born out of the battle is an opinion widely held in military and general histories of Canada. The last large-scale offensive of this period was a British attack  at Passchendaele . This offensive opened with great promise for the Allies, before bogging down in the October mud. Casualties, though disputed, were roughly equal, at some 200,000-400,000 per side. The years of trench warfare on the Western front achieved no major exchanges of territory and, as a result, are often thought of as static and unchanging. However, throughout this period, British, French, and German tactics constantly evolved to meet new battlefield challenges.
What happened second: protracted action at Verdun or the costly Nivelle Offensive?
the costly Nivelle Offensive