Without an opportunity, their abilities would have been wasted, and without their abilities, the opportunity would have arisen in vain.

...no one should marvel at the ease with which Alexander [the Great] kept possession of Asia, or at the difficulties which others, like Pyrrhus and many more, had in preserving their conquests. The difference does not arise from the greater or lesser ability of the conqueror, but from dissimilarities in the conquered lands.