Secondly, most of the details of the alliances were kept secret. For example, the Dual Alliance was formed secretly as German was not willing to antagonize Russia. As a result, mutual suspicions and fears among the powers were deepened and the powers watched others more suspiciously. Tension between the two military camps was further increased.
Thirdly, because of the assurance provided by the alliance system, the European powers believe that their allies would give them military support if war broke out, so they became less willing to settle disputes by peaceful means.
Fourthly, the two camps felt the urge to be stronger that the other side, thus intensifying the armaments race and heightening the expansion of the size of the army and the navy. For example, Britain and Germany competed in building dreadnoughts.
Fifthly, any quarrel between countries within the 2 camps could easily involve all other members of the camps. For example, in the Bosnian Crisis, Germany was supporting Austria, while Russia was on Serbia’s side, making the original small quarrel between 2 countries become a larger conflict involving four countries.
Also, the nature of the alliances had changed by 1910, compared to before when the alliances were originally strictly defensive. The Dual Alliance was modified that it had become an aggressive alliance after the Bosnian Crisis in 1909, the German government promised to give military aid to Austria, if Austria invaded Serbia and Russia intervened on behalf of the matter.
The alliance system led directly to the WWI when it turned the quarrel between Austria and Serbia after the Sarajevo Assassination into a conflict between 7 countries. On June 28, 1914, Archduke Ferdinand, heir to the Austro-Hungarian Empire and his wife were assassinated by a Bosnian Serb, called Gavrilo Princip in Sarajevo. As the assassination was supported by a Serbian secret society called the Black Hand, Austria put the responsibility on Serbia. After receiving unlimited support (i.e. “blank cheque”) from Germany on July 5, Austria sent an ultimatum to Serbia on July 23. Serbia accepted most conditions but not the last one of the ultimatum on July 25, therefore Austria declared war on Serbia on July 28. On July 29, Russia ordered troops to mobilize to support Serbia, and Germany sent an ultimatum to demanding demobilization of Russian troops on July 30. Russia refused so Germany declared war on Russia, France mobilized in support of Russia on 1 Aug. On Aug 3,Germany declared war on France, Germany invaded France via Belgium, according to the Schlieffen Plan. Britain sent an ultimatum to Germany asking her to stop invading Belgium. Germany refused, so Britain declared war on Germany on Aug 4. It is obvious that the alliance system had caused all together 7 countries (Serbia, Austria, Germany, Russia, France, Belgium, Britain) into conflicts in only a short period of time.
In conclusion, the Alliance System was an important cause of the WWI because it divided the European powers into two rival military camps, the Triple Alliance and the Triple Entente, that the rivalry between the two camps brought about the war. The alliance system led directly to the outbreak of the WWI when it turned the quarrel between Serbia and Austria into a large conflict with 7 countries involved.