Wonderful advice from the people who brought you The Prince and The Art of War. Reflects the general philosophy that after you have exerted the effort to defeat someone, your children shouldn't have to. A typical approach would involve killing everyone, plundering everything valuable, burning everything burnable, reducing the buildings to rubble, crushing the rubble into gravel, grinding the gravel into sand, baking the sand until it glows, then spreading salt (or nuclear waste) on the ground so that nothing ever grows there again.
Good historical examples would be the last Punic War, the Mongol Invasion of Russia, and General William T. Sherman's March to The Sea.
Good examples of the dangers of not following this policy first two Punic Wars, World War I, The Hundred Years War and every war that America has fought in the last 50 years.
In recent decades, it has become unfashionable to follow this doctrine as it is considered inhumane and uncivilized. However, there are advantages to never having to deal with someone ever again.