In 1904, Britain, France, and Russia formed an alliance called the "Triple Entente" as a means to protect their countries from the might of the "Triple Alliance" (Germany, Austria-Hungary, and Italy.)
In 1905, German Chief of the General Staff, Count Alfred von Schlieffen devised an operational military plan to be used in any future two-front war against France and Russia. Known as the Schlieffen Plan, the plan's purpose was to defeat France and then Russia.
In 1914, to activate the Schlieffen Plan required the German army to cross neutral Belgium for quick access to Paris. Done on August 3, this German move ultimately brought the British into World War I because Germany's actions violated the 1839 Treaty of London Britain had guaranteed to Belgium.