Question

What is an anvil ? What does vin below mean when he said Germany will be a hammer or an anvil

Answer 1

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. 

Answer 2

An anvil is a heavy steel or iron block with a flat top, concave sides, and typically a pointed end, on which metal can be hammered and shaped.