Why did the First World War create a dilemma for European socialists? How did the various factions respond to the war?

The outbreak of war in 1914 created a huge dilemma for European socialists. If they supported the war they could be accused of going back on their internationalist principles, effectively rejecting the necessity of working-class solidarity against a common class enemy. On the other hand, if they opposed the war, they ran the risk of being politically marginalized, alienated from the overwhelmingly patriotic working classes whose interests they were supposed to represent.
This dilemma manifested itself in two general attitudes. The first was held by the majority and was exemplified by Europe's largest socialist party, the German Social Democrats (SPD). These socialists wholeheartedly supported the war, putting aside their class antagonisms to embrace nationalism. The working-classes of Europe were overwhelmingly patriotic and rallied to the cause with as much enthusiasm as any other group in society. They also saw their participation in the conflict as a chance to prove they were worthy of additional civil and political rights. At the outbreak of war, the German Kaiser famously declared that he recognized no classes, only Germans. Whatever the sincerity of this statement, there's little doubt that it captured the prevailing mood, especially among the working classes and their political representatives.
A minority of socialists, however, were profoundly hostile to the war from the very start. As far as they were concerned, this was an imperialist war, a conflict fought between the ruling houses of Europe. As such, it had nothing to do with the working classes. Radical socialists saw the First World War as an opportunity to bring down what they saw as a system on the brink of collapse. They believed that the European working-classes should come together and fight the ruling classes instead of killing each other on the field of battle, serving as little more than cannon-fodder in a war which was ultimately not in their class interests.