For reasons that elude me, the precepts and principles of war taught in college introductory military history courses (my college's ROTC) never seem to take root at the higher levels of policy making. If it did, politicians and generals would insist on having a grand strategy, a theater strategy, an operational level plan, and a set of objective benchmarks for success or failure (or in between). Yet it seems we have gone in without having much strategy or planning at all - at least beyond the immediate operational needs.
Nor does it seem that the institutional lessons learned are lasting, limited to recalling those lessons acquired in the last Gulf war (1990), but no earlier. Vietnam taught a lot of hard lessons about insurgency warfare between 1964 and 1o 1969, one of them being the pointlessness of using body counts from search and destroy missions as a benchmark for success, rather than focusing on winning the hearts and minds of innocent and terrified civilians in such a war.
In short, the US belatedly accepted in 1969 that the British already had already shown how to successfully fight counter-insurgencies in Malaysia - you must clear areas and protect. Only by protecting villages and populations from retribution can trust be established and eventually give confidence in the population to join militias and cooperate. Leaving a population open to retribution after the American forces leave is a good way to never succeed. (Note, after 69 the VC was effectively neutered so the NVA took over as a conventional force and the nature of the war changed).
Yet, in spite of these learnings, the allied occupying forces in Iraq were clueless. The General that took over at the beginning of the Iraq war of occupation, Gen. John Abizaid, wasn't old enough to be in Vietnam, and his combat experience was limited to Grenada and the Gulf War. Hence, a generation of officers if they were taught counter-insurgency doctrine they never had the vision or leadership in Iraq to use it properly - not until after three years of insurgency, in 2006, a field commander named McMaster revitalized the doctrine and made it effective - to everyone's amazement.
When it comes to strategy and operational planning, no war should be entered into without a clear grasp of realistic objectives and costs, as well as a pre-determined red line where it is no longer worth fighting. And if one is to fight a war, one does not dither and oscillate over the employment of force. You hit as hard and fast as you can limited only by what you know you will never do. If you fail, you will know it quite early and you don't stick around, having done your best you withdraw or sue for peace.
Half-measures rarely work and only prolong the agony of war.