December 1915-January 1916: A German spy in Russia reveals the strain on the Russian economy to Germany by noting the unrest and general atmosphere in St. Petersburg. A report noting this reaches Erich von Falkenhayn's desk on January 24th, and convinces Falkenhayn that Russia might be able to be knocked out of the war in the near future if given some "motivation." With this new information, and the support of many other officials who favor an Eastern Strategy, he makes the fateful decision to cancel the upcoming Verdun offensive, and redirect its forces to Russia in an attempt to finally break its back.
May 30th, 1916: The offensive in the East begins after a period of redeployment, planning, and waiting for good weather. Immediately, the weakness of the Russian Army is shown as massive gains are made, most notably including the capture of Riga, and the collapse of the front line there. The planned Brusilov Offensive is delayed, then cancelled after just 5 days, due to the loss of key staging areas, including Rovno and Tarnopol. Instead, the forces are redirected towards a defensive operation to restore the front line in the North.
June 23rd: The British and French armies prematurely launch their offensive at the Somme in a desperate attempt to relieve the pressure on Russia. It initially fails miserably, with French troops simply refusing to make what they consider suicidal attacks after just 2 days, and the British troops suffering 30,000 casualties on the first day alone. However, tactics quickly change, and by July 1st, the Germans are forced to pull back troops from the Eastern Offensive as they are pushed back nearly a mile from their pre-offensive front lines. Still, the damage is done, as there is ever more unrest in Russia, and the country looks to be increasingly unstable.
July 5th: Romania and the Entente bilaterally reject the Treaty of Bucharest as Kiev falls to Austro-Hungarian forces. Romania doesn't wish to be involved in a losing war against a seemingly unstoppable Germany, and the Entente can't afford to send significant assistance to a weak ally with the current crisis in Russia.
August 7th: the Tsar abdicates and an interim government is formed as riots and unrest reach critical levels, although Russia remains in the war to honor her alliances for the time being. Still, Germany, at von Falkenhayn's advice, offers peace terms to Russia, including the loss of Poland and the Baltic States to German influence, minor territorial cessions to Austria-Hungary, and the cession of territory south of the Caucasus to the Ottoman Empire, including oil-rich Baku. While it does not include German occupation or war reparations to be paid, there are a number of economic conditions designed to ease the pressure on Germany's own war economy.
September 1916-March 1917: The front lines solidify as the winter hinders offensive movement and logistics. Still, however, the Russian Interim government is beset on all sides by strikes and mutinies against the ever more unpopular war. It is apparent that they will not be able to offer significant resistance to a 1917 offensive by Germany, Austria-Hungary, or even the Ottoman Empire.
October 1916: Following the inconclusive Battle of Jutland, it is decided (with the support of Falkenhayn) to recommence unrestricted submarine warfare beginning in late January 1917. This is hoped to destroy Britain's ability to feed herself and thus force a peace with her before the US could get involved. Combined with the increasingly obvious collapse of Russia, this would isolate France and likely force her out of the war as well.
March 25, 1917: The US declares war on Germany as a result of the Zimmerman Telegram and unrestricted submarine warfare, much earlier than Germany predicted.
March 30, 1917: the Russian Provisional Government agrees to the German terms at the Treaty of Brest-Litovsk, which frees up German troops for use on the Western, Balkan, Italian, and Middle Eastern Fronts. Ironically, though it is a great improvement for Germany's position in the war, it ironically strengthens Britain and France's ability to fight the war as they no longer have to send considerable aid to Russia. Combined with the might of the United States entering the war, Falkenhayn has no illusions about the difficulty of finally defeating a US-backed France even if Britain is forced out due to the submarine blockade.