The essential goals of warfare have not changed, but the way wars are waged certainly have. A hundred years ago, men marched to battle, lined up against one another, easily identifiable in their uniforms, and used weapons that, for the most part, would only work "up close and personal." While there were some longer-range weapons, the cannon, for example, they were not very long range, and people who killed one another were in close proximity. As time went on, longer-range weapons emerged, and people began to be able to kill one another from greater and greater distances. Today, the United States has the capability of decimating towns without having to have a person within a thousand miles. When people can kill one another without seeing one another, this is an essential change in the nature of warfare.
In more recent years, we have seen the advent of more "guerilla" warfare, in which the enemy is not easily identifiable because there are no uniforms and no "front lines." Vietnam might have been the first such "modern" war. Today, the war in Afghanistan is a good example.
Another difference, a more recent one, is the inability to identify an enemy because the enemy is not a national entity. This is the essence of terrorism, I think. When we are at war against another country, there are international conventions that all must at least theoretically adhere to. There can be negotiation for peace, treaties over land. But when the enemy comprises people from multiple countries, this is not the case, and this is a major difference that is recent.
PS... In one way, war hasn't changed at all for thousands of years. Throughout history there is just one thing that affects the outcome of war more than any other... money.
Wars cost lots of money. It is more than likely that the richest side will win in a conventional war.
The most significant change in warfare in the last hundred years is the development of the military aircraft. The main objective of any conventional modern war is air supremacy. All other things are secondary. In a nutshell, if you have air supremacy... you've won.
A typical localised US war goes like this..
The US navy moves floating airbases (aircraft carriers) to within range of the enemy and then protect those airbases from attack.
These floating airbases provide fighter bombers and fighter aircraft support while long-range bombers fly right across the world to drop incredibly destructive weapons on the enemy (carpet bombing). The carriers also fire flying 'smart' missiles to pinpoint the enemies strategic centers and military threats such as surface-to-air missiles. The airforce then spend weeks and months bombing infrastructure and military targets.
The army's job is to 'mop up' once the enemy has been critically damaged.
100 years ago, there were no military aircraft. The first were used in WW1, since then they have become the most essential part of any military planning. Aircraft have completely changed warfare in the last 100 years. They are extremely expensive and require highly-skilled operators.