Answer :

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.

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