The winter of 1916-17 seemingly froze movement in the trenches. The French and British commanders awaited the spring thaw to resume the Somme Offensive. On the German side, elaborate plans for a calculated retreat were in motion. The German High Command, feeling attrition on two fronts, wished both to strengthen and to shorten their Western front line. Work began already in September of 1916 on new fortifications along what they called the Siegfriedstellung (Hindenburg Line) -- about 25 miles east of the front line. The new defense would delay any French or English assaults in the spring of 1917. This would buy the Germans another year of trench warfare. Operation Alberich commenced on Feb 9, 1917.
Elsewhere, two ominous events would forever change the world. The first was the entry of the US into the war against Germany, triggered by Germany's resumption of unrestricted submarine warfare in February, 1917.* The second was the collapse of Imperial Russia in March of 1917, the subsequent rise of Lenin's power, and the consequent peace treaty with the Russians. The latter event allowed the Germans to fight a single-front war and to transfer men and materiel to the West. The arrival of American Doughboys bolstered the dwindling French morale and lessened the horrific casualties of the British. All in all it guaranteed another 23 months of fighting.
*The Germans had suspended unrestricted submarine warfare in 1915, after the sinking of the RMS Lusitania.