Anvil is a tool used by blacksmiths, and it is the surface on which the object is placed which is worked on with a hammer- stroked with a hammer.
von Bülow meant (in 1899) that Germany will either have to colonize others ( be the hammer) or it would have to risk being colonized itself (being the anvil), hence showing the need for Germany to participate in colonization.
In 1492, Christopher Columbus, supported by the Spanish government, undertook a voyage to find a new route to Asia and inadvertently encountered “new” lands in the Americas full of long established communities and cultures. Other European countries quickly followed suit and began to explore and invade the New World.
Shi Huangdi
Hope that helps