If others choose to accumulate resources and use them to destroy their anarchic neighbour to seize their resources, the anarchists will obviously have to defend themselves.

Accumulation of resources implies exploitation. Anarchists will agitate those exploited to overthrow their exploiters.

How do you defend yourselves against such a threat? To do so you are forced to accumulate resources.

That doesn't follow. Unless by "accumulate resources" you mean the generic practice of a society having stuff? Anarchists don't need a single person accumulating stuff to defend themselves.

I think you should read more about anarchism instead of imagining wild scenarios. Rest assured that anarchists have considered the defense argument.