Even the resumption of unrestricted submarine warfare by the Germans was not enough to push Woodrow Wilson into supporting war, but the Zimmerman Telegram made it impossible to oppose war any further. Additionally, the Russian Revolution eliminated an unsavory ally, replacing him with a fledgling democracy that needed support.
The revolutionary upheavals in Petrograd lead to the formation of a Provisional Government. Emperor Nikolai II abdicates, ending the 304-year old Romanov dynasty.
With political resentments already high, the deprivations of the harsh winter of 1916-17 cause them to boil over. The Russian Revolution has begun.
When the Great War began, Russian political factions mostly united in a common front to support the war effort, as political parties did in the other belligerent nations. But when Russia's military reversals and shortcomings in leadership became too obvious to ignore, opponents of the government began to speak up.
With the Mexican Revolution winding down and the prospects of war between Mexico and the USA seeming increasingly remote, the German Foreign Secretary explores the idea of inviting Mexico to declare war on the United States.
Germany was already rationing food when the bad harvests of 1916 made the situation far worse. Running out of options, the German military decides to resume unrestricted U-boat warfare.
The European powers refused to negotiate, but private groups, including women's groups, socialists, and Henry Ford, pressed ahead with campaigns to bring the belligerents to the negotiating table.
The automobile and the airplane, both recent inventions that make use of the internal combustion engine, become weapons of war.
With four years of peace and progressive reform and a booming economy (due to the strong French and UK wartime demand for US imports), you would think Woodrow Wilson would cruise to an easy re-election. You would be wrong.