Revolution means an overthrow of a government, usually through the use of force, such as a coup d'etat. It could also mean a scientific or artistic breakthrough.
After the Civil War, Republicans attempted to grant freedom to the slaves and with give them their rights as citizens, since they knew that the southern states were clearly democratic, in order to maintain Republican control over the defeated southern states.
Crime can affect communities in many ways, but perhaps the greatest economic impact is that more security forces are needed, which costs lots of money.
The significance of military action in WWII is that without taking this necessary action, (assuming you are referring to the U.S.) WWII could have had a very different outcome. The U.S. involvement in WWII was the cause of the bombing of Pearl Harbor. If they hadn't formed the Allied Forces to defeat Germany and Hitler, Germany very well might have won.
