Good question. Pretty much as soon as the country was founded it was considered a powerful force. It disrupted the balance of power in Europe, since there had been a loose amalgamation of Germanic states in that area for a while

In 1881 Germany hosted the Berlin Conference, in which the major powers of Europe decided to carve up Africa. Germany got colonial possessions just like England and France (and other nations)

So I would say that 1881 is when Germany became a world power