Why did the way WWII ended in the Pacific change how wars are fought in the present day?
In order to understand the answer to this, we first have to remember how the war in the Pacific ended. The war in the Pacific ended, in essence, with the dropping of the atomic bombs on Hiroshima and Nagasaki. This changed the way in which wars are fought today because it ushered in the age of nuclear weapons.
Nuclear weapons have changed the way in which wars are fought in two ways. First, it has drastically reduced the number of wars between major powers. In essence, there has been no war pitting two major powers against one another since World War II. The US and other nuclear powers have carefully avoided fighting one another because of nuclear weapons. Second, it has also caused wars to be more limited. Countries that have nuclear weapons are much less willing to fight all out wars against one another. They do not want to get to the point where one side or the other will be tempted to use its nuclear weapons.
Thus, the dawn of nuclear weapons (occurring at the end of WWII in the Pacific) reduced the number and intensity of wars involving great powers.