Final answer:
The lesson learned when it comes to rebuilding nations is that nation-building requires careful planning and resources.
Step-by-step explanation:
The text suggests that the lesson we have learned when it comes to rebuilding nations is that nation-building requires careful planning and resources. The passage mentions that newly independent states often ran into difficulties because their political institutions were weak and their economies were not developing fast enough. It also highlights the challenges faced in building nations, such as the need for educated leaders and alternative ways of organizing a state. Therefore, careful planning and allocating enough resources are necessary to successfully rebuild nations.