I think you can answer this question just by thinking about it practically. The strategic management process includes planning, implementing and evaluating. There are several reasons the implementation stage could be considered the most difficult:
- Even the best ideas on paper do not always turn out the same in practice.
- Usually the number of people in the "planning" step is only a fraction of the number of people who will be involved in carrying out the plan - anytime you increase the number of people - you increase the difficulty of a plan.
- People are not robots - even the most meticulous (or simple) directions might not be understood by everyone.
- Unforeseen obstacles inevitably come up and require instant problem-solving -or- going back to the drawing board.
- Because strategic management plans are laid out with a long-term vision in mind - they do not always work immediately - more often than not, they need time to show results. It is difficult to prove that change is a good thing, when the results are long in coming.
These are just some of the reasons why the "implementing" stage is the most difficult. Planning and evaluating may end up taking more time - but they are much more straightforward. The "implementation" is so action-oriented that it is often impossible to plan for everything that might happen.