german strategy for a war on two fronts against france and russia was to rapidly concentrate the bulk of its army in the west to defeat france within four weeks, then shift forces to the east before russia could fully mobilise; this was later known as the schlieffen plan. on 2 august, germany demanded free passage through belgium, an essential element in achieving a quick victory over france.
when this was refused, german forces invaded belgium on 3 august and declared war on france the same day; the belgian government invoked the 1839 treaty of london and in compliance with its obligations under this, britain declared war on germany on 4 august. on 12 august, britain and france also declared war on austria-hungary; on the 23rd, japan sided with the entente, seizing german possessions in china and the pacific. in november 1914, the ottoman empire entered the war on the side of the alliance, opening fronts in the caucasus, mesopotamia and the sinai peninsula. the war was fought in and drew upon each power's colonial empire as well, spreading the conflict to africa and across the globe. the entente and its allies would eventually become known as the allied powers, while the grouping of austria-hungary, germany and their allies would become known as the central powers.
the german advance into france was halted at the battle of the marne and by the end of 1914, the western front settled into a battle of attrition, marked by a long series of trench lines that changed little until 1917 (the eastern front, by contrast, was marked by much greater exchanges of territory). in 1915, italy joined the allied powers and opened a front in the alps. bulgaria joined the central powers in 1915 and greece joined the allies in 1917, expanding the war in the balkans. the united states initially remained neutral, though even while neutral it became an important supplier of war material to the allies. eventually, after the sinking of american merchant ships by german submarines, and the revelation that the germans were trying to incite mexico to make war on the united states, the u.s. declared war on germany on 6 april 1917. trained american forces would not begin arriving at the front in large numbers until mid-1918, but ultimately the american expeditionary force would reach some two million troops.
though serbia was defeated in 1915, and romania joined the allied powers in 1916 only to be defeated in 1917, none of the great powers were knocked out of the war until 1918. the 1917 february revolution in russia replaced the tsarist autocracy with the provisional government, but continuing discontent at the cost of the war led to the october revolution, the creation of the soviet socialist republic, and the signing of the treaty of brest-litovsk by the new government in march 1918, ending russia's involvement in the war. this allowed the transfer of large numbers of german troops from the east to the western front, resulting in the german march 1918 offensive. this offensive was initially successful, but failed to score a decisive victory and exhausted the last of the german reserves.
the allies rallied and drove the germans back in their hundred days offensive, a continual series of attacks to which the germans had no reply. bulgaria was the first central power to sign an armistice—the armistice of salonica on 29 september 1918. on 30 october, the ottoman empire capitulated, signing the armistice of mudros. on 4 november, the austro-hungarian empire agreed to the armistice of villa giusti. with its allies defeated, revolution at home, and the military no longer willing to fight, kaiser wilhelm abdicated on 9 november and germany signed an armistice on 11 november 1918, effectively ending the war.