In the American Civil War, the battle of Gettysburg was definitely a turning point. Before that the South was on a roll, and had the manpower to continue prosecuting it's interests in the war. It's hard to say if the South could have really defeated the North in an extremely protracted conflict, but their objectives were not to subjugate the Union, just to guarantee their independence from it. After this battle the South had lost much of it's ability to effectively continue the war.
In WW2 the question is more difficult. There are many battles that could be called definitive. One could call the Battle of Britain an important turning point. I think something on the Eastern front, whether Kursk or Stalingrad would in the end be more crucial to the fall of the Third Reich.
In the Pacific theater there were definitely turning points. While the Japanese would not have been able to hold out in a long and drawn out battle with the might of American industry, their ability to make war and their territorial possessions cannot be discounted. The battle of Midway eliminated much of their carrier force, and likewise the battle of the Philippine Sea (Marianas Turkey Shoot) eliminated much of the remaining naval air power at their disposal. It can be said that Japan was on it's haunches at that point already, and I think in the end the overall outcome of the war was indeed a foregone conclusion at the start of it, given America's new found tenacity, but Japan's ability to make war would have necessitated a much different outcome than proved to be the case. In this sense I think the battle of Midway was a definite turning point.
