This really is a complex question, because it depends on the situation so much.
I would have to echo the others who quote Clauswitz. Without a clear political objective that you can accomplish with an army, you probably should find another way to accomplish your goals. Prepare to defend yourself and your intrests to the best of your abilites, and make the threat of using your army a very real one. But don’t willingly commit your army unitl you know exactly what things you want to do with it, if you can actually help it.
I’d consider that the proper way of waging war, not just a modern one. Even then, it’s not a recipe for victory. Once you have decided to go to war, or let it come to you, you have to decide what kind of war you want it to be. To mention just one aspect of war, length: A short, decisive one? Or a long, drawn out one? If we were for example to go to war with China, we would want it to be quick, since numbers of warm bodies and years to put all of your nation’s resouces into a war effort play into their hands. If the combatants were India and China, I think both sides would still want to try to win quick. However, the advantage of a decisive attack would be less useful, since they share a border, and plenty of munitions can be delivered by foot. Not to mention, the events that would bring any combination of these nations to war would be different, so the political objectives would be different changing each situation further.
After the war, there is usually something done to keep the peace. Either withdrawing to your border with a treaty and a wary eye, or if you are really successful; occupying the place until folks are broken enough to let you run the place for them.
Afghanistan: In retrospect, I feel that we should have offered the Northern Alliance all the air support and ground weapons it could carry, with plenty of financial help afterward. I’d like to say I thought that clearly in 2001, but I can’t.
Iraq: I have no idea, the way we went in seems to have skipped the first part of the above process. We did the middle perfectly, which is exactly what the military trains itself to do. The first part and the third part are for the Politicians and the diplomats to decide, which was not done well in this instance.
The only thing worse is to do what happened in Vietnam, when the politicians were given free reign over all three stages. Then they fouled up 1 and 2, but got three, albeit a bit late.
I really don’t think that’s fair. The OP asked about the conflicts the current president presided over. The fact they see them as poorly executed is not neccessarily a partisan attack. Nonetheless, the last war that a Democrat President got us involved in hasn’t fared too well in this thread, either. Great Britan and the USSR making similar mistakes have been mentioned as examples as well. If you have a theory on how modern war should be waged, say it.