The German kaiser was horrified at the assassination of a royal heir. He assured the Austrian-Hungarian emperor of Germany’s full support. Thus, instead of urging restraint, Germany gave a Austria a “blank check.”
Austria-Hungary reponsed to the assassination by issuing an ultimatum, or final set of demands.
The assassination of Archduke Francis Ferdinand of Austria-Hungary by Serbian nationalist who resented Austrian rule was the "spark" of World War I.
WE declare WAR on Germany!!
The Russian czar asked the German kaiser to urge Austria to soften its demands. When this plea failed, Russia began to mobilize, or prepare its military forces for war. Germany responded by declaring war on Russia.
When Germany launched the Schlieffen Plan, Invading Belgium as the beginning of the plan to conquer France and avoid a two front war with France and Russia, Britain declared war on Germany.
"There will be war!!"
France tried to help be got rejected. When Germany demanded that France keep out of the conflict, France refused. Germany then declared war on France.