and German forces unsuccessfully tried to outflank each other, a series of manoeuvres later known as the "Race to the Sea". By the end of 1914, the opposing forces confronted each other along an uninterrupted line of entrenched positions from the Channel to the Swiss border. Since the Germans were normally able to choose where to stand, they generally held the high ground, while their trenches tended to be better built; those constructed by the French and English were initially considered "temporary", only needed until an offensive would smash the German defences. Both sides tried to break the stalemate using scientific and technological advances. On 22 April 1915, at the Second Battle of Ypres, the Germans (violating the Hague Convention) used chlorine gas for the first time on the Western Front. Several types of gas soon became widely used by both sides, and though it never proved a decisive, battle-winning weapon, it became one of the most-feared and best-remembered horrors of the war. Continuation of trench warfare Neither side proved able to deliver a decisive blow for the next two years. Throughout 1915–17, the British Empire and France suffered more casualties than Germany, because of both the strategic and tactical stances chosen by the sides. Strategically, while the Germans mounted only one major offensive, the Allies made several attempts to break through the German lines. In February 1916 the Germans attacked French defensive positions at the Battle of Verdun, lasting until December 1916. The Germans made initial gains, before French counter-attacks returned matters to near their starting point. Casualties were greater for the French, but the Germans bled heavily as well, with anywhere from 700,000 to 975,000 casualties suffered between the two combatants. Verdun became a symbol of French determination and self-sacrifice. The Battle of the Somme was an Anglo-French offensive of July to November 1916. The opening day on 1 July 1916 was the bloodiest single day in the history of the British Army, which suffered 57,470 casualties, including 19,240 dead. As a whole, the Somme offensive led to an estimated 420,000 British casualties, along with 200,000 French and 500,000 German. Gun fire was not the only factor taking lives; the diseases that emerged in the trenches were a major killer on both sides. The living conditions made it so that countless diseases and infections occurred, such as trench foot, shell shock, blindness/burns from mustard gas, lice, trench fever,