Some things I have found. Most are obvious.
1) All wars are unique. Like fingerprints. Many similarities can and do exist between wars, however their are ALWAYS significant differences. One common mistake (Generals are only a little less likely to make this mistake as anyone else) that is to fight the prior war again. A classic example: The construction of the Maginot line in the 1930's by France.
2) Violence goes up as wars progress. Lot's of reasons. The winner in war is determined by who is willing and able to raise the violence to a level that the other side either cannot or will not match.
3) Three levels of viewing warfare:
a) Strategic: View of entire continent or oceans. The conflict overall. The BIG picture.
b) Operational: View of individual armies and/or fleets. Divisional level and above.
c) Tactical: View of individual soldier up to divisional level. Or task force in the navy.
4) Two basic types of warfare:
a) War of conquest. Take care of the population. You want them to join you.
b) Punitive. Wipe everyone and everything out. You do not intend to stick around.
5) Political considerations are generally counterproductive in warfare. Many examples of entire armies being wiped out and wars lost because of political considerations.
6) Occupation: The worst enemy of the occupied is NOT the occupier. It is the population of the occupied that is working with the occupier.
7) Wars ALWAYS create new enemies.
8) Atrocities occur in all wars. The vast majority goes unreported for obivious reasons. One of the cleanest wars in modern history was the U.S. and England against Germany. (1939-1945) Atrocities occurred even then, on both sides. A direct relationship exists between number of atrocities and the training that a unit has undergone. The more training, the fewer atrocities committed by that unit. This applies to armies as well. (Generally)
9) Cultural wars are the most brutal. What happend to the native Americans is a classic example. Not only is language a difference, but the very habits that a population have. In war, it is necessary to dehumanize your enemy. For example, you shoot down an enemy plane or sink an enemy ship. You tell yourself that it was only a machine. You attempt to deny that it had people in it. This is a natural self-defense mechanism. Cultural difference make this easier.
"A good Jap is a dead Jap." For a number of reasons, when you found a wounded Japanese, you put a bullet in his head. Just to be sure. One potential side-effect of allowing suspected terrorists the same legal rights that U.S. citizens enjoy is an increase in the number of killing of potential prisioners. Many of the same motivations for "A good Jap is a dead Jap." exist today in the war in Iraq. Vietnam had many of the same motivations as well.
10) The longer a seige lasts, the more brutal the final outcome. In general, this applies to wars as well.
11) Inferior allied troops perform much better when mixed with more effective troops. The war in North Africa 1940-43 is a good example. German troops were much better than Italian. When mixed, Italian troops performed almost as well as the German, and the German troops who were mixed with them did not suffer any noticable ill-effects. I believe that the Roman army used this pricipal for centuries with impressive results. The rise of the nation-state has tended to install the idea that it is better to separate them, for nationalist reasons. This has been proven time and again to be a mistake.