What military alliances pushed European nations into war? (World War I and World War II)

In World War I, there were two alliances that pushed the world into war.  These alliances were the Triple Alliance and the Triple Entente.  The alliances did not cause the war to begin, but when the war did start, they caused it to spread farther than it might otherwise have done.

World War I started when Austria-Hungary declared war on Serbia.  This came after a Serbian nationalist named Gavril Princip assassinated Archduke Franz Ferdinand of Austria.  Austria blamed Serbia for the attack and issued an ultimatum that Serbia was sure to reject, thus bringing about war.  So, the war began not because of alliances, but because of nationalist opposition to the Austro-Hungarian Empire.

However, the war spread because of alliances.  Austria-Hungary was part of the Triple Alliance.  They joined Italy and Germany in this alliance.  They were opposed by the Triple Entente, which was made up of Britain, France, and Russia.  Russia was allied with Serbia because both were Slavic countries and they felt the need to be allied against the ethnic German countries (Germany and Austria).  When Serbia and Austria-Hungary went to war, Russia felt compelled to enter the war on Serbia’s side and Germany entered on Austria’s side.  This activated the system of alliances and widened the war dramatically.  In this way, the existence of these alliances broadened the war, even if it did not cause the war.

It is somewhat harder to say that alliances pushed European nations into World War II.   WWII was fought by two main alliances, the Axis and the Allies.  However, these alliances were not really what pushed the European countries into the war.

It is more accurate to say that Hitler pushed European nations into WWII when he finally went too far and forced those nations to fight.  Britain and France declared war on Germany when Germany invaded Poland.  You could say that this was because they had alliances with Poland, but it was really because they finally decided that Hitler was a major menace to their security.  They fought to protect themselves, not for the sake of Poland, which was beyond their help in any case.  Hitler did not fight because of his alliance with Italy.  He fought to make his country more powerful.

When the USSR entered the war, it did so because Hitler attacked it.  In fact, the USSR and Germany had nominally been allies (or at least had a non-aggression pact) right up until Germany invaded.  Clearly, the USSR did not join the war because of an alliance.  Thus, it is difficult to argue that military alliances pushed European countries into WWII.