WWI (or the prelude to it) divided Europe into the two groups of allies that you mention. It did this because the prelude to the war involved countries feeling that they needed help in protecting themselves from their enemies.
In the prelude to WWI, Germany was noted for its attempts to gain power. This helped to induce other countries to ally themselves against Germany. This is what led to the creation of the Triple Entente. Confronted with this alliance, Germany, too, sought allies.
The prelude to WWI, then, divided Europe by forcing various countries to take sides as a way of trying to protect themselves.